762 lines
23 KiB
ReStructuredText
762 lines
23 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Journal (jbd2)
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--------------
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Introduced in ext3, the ext4 filesystem employs a journal to protect the
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filesystem against metadata inconsistencies in the case of a system crash. Up
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to 10,240,000 file system blocks (see man mke2fs(8) for more details on journal
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size limits) can be reserved inside the filesystem as a place to land
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“important” data writes on-disk as quickly as possible. Once the important
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data transaction is fully written to the disk and flushed from the disk write
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cache, a record of the data being committed is also written to the journal. At
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some later point in time, the journal code writes the transactions to their
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final locations on disk (this could involve a lot of seeking or a lot of small
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read-write-erases) before erasing the commit record. Should the system
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crash during the second slow write, the journal can be replayed all the
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way to the latest commit record, guaranteeing the atomicity of whatever
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gets written through the journal to the disk. The effect of this is to
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guarantee that the filesystem does not become stuck midway through a
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metadata update.
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For performance reasons, ext4 by default only writes filesystem metadata
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through the journal. This means that file data blocks are /not/
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guaranteed to be in any consistent state after a crash. If this default
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guarantee level (``data=ordered``) is not satisfactory, there is a mount
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option to control journal behavior. If ``data=journal``, all data and
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metadata are written to disk through the journal. This is slower but
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safest. If ``data=writeback``, dirty data blocks are not flushed to the
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disk before the metadata are written to disk through the journal.
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In case of ``data=ordered`` mode, Ext4 also supports fast commits which
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help reduce commit latency significantly. The default ``data=ordered``
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mode works by logging metadata blocks to the journal. In fast commit
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mode, Ext4 only stores the minimal delta needed to recreate the
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affected metadata in fast commit space that is shared with JBD2.
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Once the fast commit area fills in or if fast commit is not possible
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or if JBD2 commit timer goes off, Ext4 performs a traditional full commit.
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A full commit invalidates all the fast commits that happened before
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it and thus it makes the fast commit area empty for further fast
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commits. This feature needs to be enabled at mkfs time.
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The journal inode is typically inode 8. The first 68 bytes of the
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journal inode are replicated in the ext4 superblock. The journal itself
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is normal (but hidden) file within the filesystem. The file usually
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consumes an entire block group, though mke2fs tries to put it in the
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middle of the disk.
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All fields in jbd2 are written to disk in big-endian order. This is the
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opposite of ext4.
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NOTE: Both ext4 and ocfs2 use jbd2.
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The maximum size of a journal embedded in an ext4 filesystem is 2^32
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blocks. jbd2 itself does not seem to care.
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Layout
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~~~~~~
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Generally speaking, the journal has this format:
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.. list-table::
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:widths: 16 48 16
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:header-rows: 1
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* - Superblock
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- descriptor_block (data_blocks or revocation_block) [more data or
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revocations] commmit_block
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- [more transactions...]
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* -
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- One transaction
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-
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Notice that a transaction begins with either a descriptor and some data,
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or a block revocation list. A finished transaction always ends with a
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commit. If there is no commit record (or the checksums don't match), the
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transaction will be discarded during replay.
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External Journal
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~~~~~~~~~~~~~~~~
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Optionally, an ext4 filesystem can be created with an external journal
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device (as opposed to an internal journal, which uses a reserved inode).
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In this case, on the filesystem device, ``s_journal_inum`` should be
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zero and ``s_journal_uuid`` should be set. On the journal device there
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will be an ext4 super block in the usual place, with a matching UUID.
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The journal superblock will be in the next full block after the
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superblock.
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.. list-table::
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:widths: 12 12 12 32 12
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:header-rows: 1
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* - 1024 bytes of padding
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- ext4 Superblock
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- Journal Superblock
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- descriptor_block (data_blocks or revocation_block) [more data or
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revocations] commmit_block
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- [more transactions...]
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* -
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-
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-
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- One transaction
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-
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Block Header
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~~~~~~~~~~~~
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Every block in the journal starts with a common 12-byte header
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``struct journal_header_s``:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Description
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* - 0x0
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- __be32
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- h_magic
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- jbd2 magic number, 0xC03B3998.
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* - 0x4
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- __be32
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- h_blocktype
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- Description of what this block contains. See the jbd2_blocktype_ table
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below.
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* - 0x8
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- __be32
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- h_sequence
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- The transaction ID that goes with this block.
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.. _jbd2_blocktype:
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The journal block type can be any one of:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 1
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- Descriptor. This block precedes a series of data blocks that were
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written through the journal during a transaction.
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* - 2
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- Block commit record. This block signifies the completion of a
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transaction.
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* - 3
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- Journal superblock, v1.
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* - 4
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- Journal superblock, v2.
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* - 5
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- Block revocation records. This speeds up recovery by enabling the
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journal to skip writing blocks that were subsequently rewritten.
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Super Block
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~~~~~~~~~~~
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The super block for the journal is much simpler as compared to ext4's.
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The key data kept within are size of the journal, and where to find the
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start of the log of transactions.
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The journal superblock is recorded as ``struct journal_superblock_s``,
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which is 1024 bytes long:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Description
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* -
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-
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-
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- Static information describing the journal.
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* - 0x0
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- journal_header_t (12 bytes)
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- s_header
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- Common header identifying this as a superblock.
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* - 0xC
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- __be32
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- s_blocksize
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- Journal device block size.
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* - 0x10
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- __be32
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- s_maxlen
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- Total number of blocks in this journal.
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* - 0x14
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- __be32
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- s_first
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- First block of log information.
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* -
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-
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-
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- Dynamic information describing the current state of the log.
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* - 0x18
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- __be32
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- s_sequence
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- First commit ID expected in log.
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* - 0x1C
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- __be32
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- s_start
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- Block number of the start of log. Contrary to the comments, this field
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being zero does not imply that the journal is clean!
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* - 0x20
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- __be32
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- s_errno
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- Error value, as set by jbd2_journal_abort().
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* -
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-
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-
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- The remaining fields are only valid in a v2 superblock.
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* - 0x24
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- __be32
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- s_feature_compat;
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- Compatible feature set. See the table jbd2_compat_ below.
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* - 0x28
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- __be32
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- s_feature_incompat
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- Incompatible feature set. See the table jbd2_incompat_ below.
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* - 0x2C
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- __be32
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- s_feature_ro_compat
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- Read-only compatible feature set. There aren't any of these currently.
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* - 0x30
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- __u8
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- s_uuid[16]
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- 128-bit uuid for journal. This is compared against the copy in the ext4
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super block at mount time.
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* - 0x40
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- __be32
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- s_nr_users
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- Number of file systems sharing this journal.
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* - 0x44
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- __be32
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- s_dynsuper
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- Location of dynamic super block copy. (Not used?)
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* - 0x48
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- __be32
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- s_max_transaction
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- Limit of journal blocks per transaction. (Not used?)
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* - 0x4C
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- __be32
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- s_max_trans_data
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- Limit of data blocks per transaction. (Not used?)
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* - 0x50
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- __u8
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- s_checksum_type
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- Checksum algorithm used for the journal. See jbd2_checksum_type_ for
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more info.
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* - 0x51
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- __u8[3]
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- s_padding2
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-
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* - 0x54
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- __be32
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- s_num_fc_blocks
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- Number of fast commit blocks in the journal.
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* - 0x58
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- __be32
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- s_head
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- Block number of the head (first unused block) of the journal, only
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up-to-date when the journal is empty.
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* - 0x5C
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- __u32
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- s_padding[40]
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-
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* - 0xFC
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- __be32
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- s_checksum
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- Checksum of the entire superblock, with this field set to zero.
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* - 0x100
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- __u8
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- s_users[16*48]
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- ids of all file systems sharing the log. e2fsprogs/Linux don't allow
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shared external journals, but I imagine Lustre (or ocfs2?), which use
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the jbd2 code, might.
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.. _jbd2_compat:
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The journal compat features are any combination of the following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- Journal maintains checksums on the data blocks.
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(JBD2_FEATURE_COMPAT_CHECKSUM)
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.. _jbd2_incompat:
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The journal incompat features are any combination of the following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- Journal has block revocation records. (JBD2_FEATURE_INCOMPAT_REVOKE)
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* - 0x2
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- Journal can deal with 64-bit block numbers.
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(JBD2_FEATURE_INCOMPAT_64BIT)
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* - 0x4
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- Journal commits asynchronously. (JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)
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* - 0x8
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- This journal uses v2 of the checksum on-disk format. Each journal
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metadata block gets its own checksum, and the block tags in the
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descriptor table contain checksums for each of the data blocks in the
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journal. (JBD2_FEATURE_INCOMPAT_CSUM_V2)
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* - 0x10
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- This journal uses v3 of the checksum on-disk format. This is the same as
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v2, but the journal block tag size is fixed regardless of the size of
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block numbers. (JBD2_FEATURE_INCOMPAT_CSUM_V3)
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* - 0x20
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- Journal has fast commit blocks. (JBD2_FEATURE_INCOMPAT_FAST_COMMIT)
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.. _jbd2_checksum_type:
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Journal checksum type codes are one of the following. crc32 or crc32c are the
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most likely choices.
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 1
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- CRC32
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* - 2
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- MD5
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* - 3
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- SHA1
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* - 4
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- CRC32C
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Descriptor Block
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~~~~~~~~~~~~~~~~
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The descriptor block contains an array of journal block tags that
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describe the final locations of the data blocks that follow in the
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journal. Descriptor blocks are open-coded instead of being completely
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described by a data structure, but here is the block structure anyway.
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Descriptor blocks consume at least 36 bytes, but use a full block:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Descriptor
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* - 0x0
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- journal_header_t
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- (open coded)
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- Common block header.
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* - 0xC
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- struct journal_block_tag_s
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- open coded array[]
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- Enough tags either to fill up the block or to describe all the data
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blocks that follow this descriptor block.
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Journal block tags have any of the following formats, depending on which
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journal feature and block tag flags are set.
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If JBD2_FEATURE_INCOMPAT_CSUM_V3 is set, the journal block tag is
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defined as ``struct journal_block_tag3_s``, which looks like the
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following. The size is 16 or 32 bytes.
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Descriptor
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* - 0x0
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- __be32
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- t_blocknr
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- Lower 32-bits of the location of where the corresponding data block
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should end up on disk.
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* - 0x4
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- __be32
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- t_flags
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- Flags that go with the descriptor. See the table jbd2_tag_flags_ for
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more info.
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* - 0x8
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- __be32
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- t_blocknr_high
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- Upper 32-bits of the location of where the corresponding data block
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should end up on disk. This is zero if JBD2_FEATURE_INCOMPAT_64BIT is
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not enabled.
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* - 0xC
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- __be32
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- t_checksum
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- Checksum of the journal UUID, the sequence number, and the data block.
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* -
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-
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-
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- This field appears to be open coded. It always comes at the end of the
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tag, after t_checksum. This field is not present if the "same UUID" flag
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is set.
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* - 0x8 or 0xC
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- char
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- uuid[16]
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- A UUID to go with this tag. This field appears to be copied from the
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``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
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field.
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.. _jbd2_tag_flags:
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The journal tag flags are any combination of the following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- On-disk block is escaped. The first four bytes of the data block just
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happened to match the jbd2 magic number.
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* - 0x2
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- This block has the same UUID as previous, therefore the UUID field is
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omitted.
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* - 0x4
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- The data block was deleted by the transaction. (Not used?)
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* - 0x8
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- This is the last tag in this descriptor block.
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If JBD2_FEATURE_INCOMPAT_CSUM_V3 is NOT set, the journal block tag
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is defined as ``struct journal_block_tag_s``, which looks like the
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following. The size is 8, 12, 24, or 28 bytes:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Descriptor
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* - 0x0
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- __be32
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- t_blocknr
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- Lower 32-bits of the location of where the corresponding data block
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should end up on disk.
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* - 0x4
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- __be16
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- t_checksum
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- Checksum of the journal UUID, the sequence number, and the data block.
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Note that only the lower 16 bits are stored.
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* - 0x6
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- __be16
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- t_flags
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- Flags that go with the descriptor. See the table jbd2_tag_flags_ for
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more info.
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* -
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-
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-
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- This next field is only present if the super block indicates support for
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64-bit block numbers.
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* - 0x8
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- __be32
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- t_blocknr_high
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- Upper 32-bits of the location of where the corresponding data block
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should end up on disk.
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* -
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-
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-
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- This field appears to be open coded. It always comes at the end of the
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tag, after t_flags or t_blocknr_high. This field is not present if the
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"same UUID" flag is set.
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* - 0x8 or 0xC
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- char
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- uuid[16]
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- A UUID to go with this tag. This field appears to be copied from the
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``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
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field.
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If JBD2_FEATURE_INCOMPAT_CSUM_V2 or
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JBD2_FEATURE_INCOMPAT_CSUM_V3 are set, the end of the block is a
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``struct jbd2_journal_block_tail``, which looks like this:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Descriptor
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* - 0x0
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- __be32
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- t_checksum
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- Checksum of the journal UUID + the descriptor block, with this field set
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to zero.
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Data Block
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~~~~~~~~~~
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In general, the data blocks being written to disk through the journal
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are written verbatim into the journal file after the descriptor block.
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However, if the first four bytes of the block match the jbd2 magic
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number then those four bytes are replaced with zeroes and the “escaped”
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flag is set in the descriptor block tag.
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Revocation Block
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~~~~~~~~~~~~~~~~
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A revocation block is used to prevent replay of a block in an earlier
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transaction. This is used to mark blocks that were journalled at one
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time but are no longer journalled. Typically this happens if a metadata
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block is freed and re-allocated as a file data block; in this case, a
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journal replay after the file block was written to disk will cause
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corruption.
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**NOTE**: This mechanism is NOT used to express “this journal block is
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superseded by this other journal block”, as the author (djwong)
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mistakenly thought. Any block being added to a transaction will cause
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the removal of all existing revocation records for that block.
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Revocation blocks are described in
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``struct jbd2_journal_revoke_header_s``, are at least 16 bytes in
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length, but use a full block:
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|
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.. list-table::
|
|
:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Description
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* - 0x0
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- journal_header_t
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- r_header
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- Common block header.
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* - 0xC
|
|
- __be32
|
|
- r_count
|
|
- Number of bytes used in this block.
|
|
* - 0x10
|
|
- __be32 or __be64
|
|
- blocks[0]
|
|
- Blocks to revoke.
|
|
|
|
After r_count is a linear array of block numbers that are effectively
|
|
revoked by this transaction. The size of each block number is 8 bytes if
|
|
the superblock advertises 64-bit block number support, or 4 bytes
|
|
otherwise.
|
|
|
|
If JBD2_FEATURE_INCOMPAT_CSUM_V2 or
|
|
JBD2_FEATURE_INCOMPAT_CSUM_V3 are set, the end of the revocation
|
|
block is a ``struct jbd2_journal_revoke_tail``, which has this format:
|
|
|
|
.. list-table::
|
|
:widths: 8 8 24 40
|
|
:header-rows: 1
|
|
|
|
* - Offset
|
|
- Type
|
|
- Name
|
|
- Description
|
|
* - 0x0
|
|
- __be32
|
|
- r_checksum
|
|
- Checksum of the journal UUID + revocation block
|
|
|
|
Commit Block
|
|
~~~~~~~~~~~~
|
|
|
|
The commit block is a sentry that indicates that a transaction has been
|
|
completely written to the journal. Once this commit block reaches the
|
|
journal, the data stored with this transaction can be written to their
|
|
final locations on disk.
|
|
|
|
The commit block is described by ``struct commit_header``, which is 32
|
|
bytes long (but uses a full block):
|
|
|
|
.. list-table::
|
|
:widths: 8 8 24 40
|
|
:header-rows: 1
|
|
|
|
* - Offset
|
|
- Type
|
|
- Name
|
|
- Descriptor
|
|
* - 0x0
|
|
- journal_header_s
|
|
- (open coded)
|
|
- Common block header.
|
|
* - 0xC
|
|
- unsigned char
|
|
- h_chksum_type
|
|
- The type of checksum to use to verify the integrity of the data blocks
|
|
in the transaction. See jbd2_checksum_type_ for more info.
|
|
* - 0xD
|
|
- unsigned char
|
|
- h_chksum_size
|
|
- The number of bytes used by the checksum. Most likely 4.
|
|
* - 0xE
|
|
- unsigned char
|
|
- h_padding[2]
|
|
-
|
|
* - 0x10
|
|
- __be32
|
|
- h_chksum[JBD2_CHECKSUM_BYTES]
|
|
- 32 bytes of space to store checksums. If
|
|
JBD2_FEATURE_INCOMPAT_CSUM_V2 or JBD2_FEATURE_INCOMPAT_CSUM_V3
|
|
are set, the first ``__be32`` is the checksum of the journal UUID and
|
|
the entire commit block, with this field zeroed. If
|
|
JBD2_FEATURE_COMPAT_CHECKSUM is set, the first ``__be32`` is the
|
|
crc32 of all the blocks already written to the transaction.
|
|
* - 0x30
|
|
- __be64
|
|
- h_commit_sec
|
|
- The time that the transaction was committed, in seconds since the epoch.
|
|
* - 0x38
|
|
- __be32
|
|
- h_commit_nsec
|
|
- Nanoseconds component of the above timestamp.
|
|
|
|
Fast commits
|
|
~~~~~~~~~~~~
|
|
|
|
Fast commit area is organized as a log of tag length values. Each TLV has
|
|
a ``struct ext4_fc_tl`` in the beginning which stores the tag and the length
|
|
of the entire field. It is followed by variable length tag specific value.
|
|
Here is the list of supported tags and their meanings:
|
|
|
|
.. list-table::
|
|
:widths: 8 20 20 32
|
|
:header-rows: 1
|
|
|
|
* - Tag
|
|
- Meaning
|
|
- Value struct
|
|
- Description
|
|
* - EXT4_FC_TAG_HEAD
|
|
- Fast commit area header
|
|
- ``struct ext4_fc_head``
|
|
- Stores the TID of the transaction after which these fast commits should
|
|
be applied.
|
|
* - EXT4_FC_TAG_ADD_RANGE
|
|
- Add extent to inode
|
|
- ``struct ext4_fc_add_range``
|
|
- Stores the inode number and extent to be added in this inode
|
|
* - EXT4_FC_TAG_DEL_RANGE
|
|
- Remove logical offsets to inode
|
|
- ``struct ext4_fc_del_range``
|
|
- Stores the inode number and the logical offset range that needs to be
|
|
removed
|
|
* - EXT4_FC_TAG_CREAT
|
|
- Create directory entry for a newly created file
|
|
- ``struct ext4_fc_dentry_info``
|
|
- Stores the parent inode number, inode number and directory entry of the
|
|
newly created file
|
|
* - EXT4_FC_TAG_LINK
|
|
- Link a directory entry to an inode
|
|
- ``struct ext4_fc_dentry_info``
|
|
- Stores the parent inode number, inode number and directory entry
|
|
* - EXT4_FC_TAG_UNLINK
|
|
- Unlink a directory entry of an inode
|
|
- ``struct ext4_fc_dentry_info``
|
|
- Stores the parent inode number, inode number and directory entry
|
|
|
|
* - EXT4_FC_TAG_PAD
|
|
- Padding (unused area)
|
|
- None
|
|
- Unused bytes in the fast commit area.
|
|
|
|
* - EXT4_FC_TAG_TAIL
|
|
- Mark the end of a fast commit
|
|
- ``struct ext4_fc_tail``
|
|
- Stores the TID of the commit, CRC of the fast commit of which this tag
|
|
represents the end of
|
|
|
|
Fast Commit Replay Idempotence
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Fast commits tags are idempotent in nature provided the recovery code follows
|
|
certain rules. The guiding principle that the commit path follows while
|
|
committing is that it stores the result of a particular operation instead of
|
|
storing the procedure.
|
|
|
|
Let's consider this rename operation: 'mv /a /b'. Let's assume dirent '/a'
|
|
was associated with inode 10. During fast commit, instead of storing this
|
|
operation as a procedure "rename a to b", we store the resulting file system
|
|
state as a "series" of outcomes:
|
|
|
|
- Link dirent b to inode 10
|
|
- Unlink dirent a
|
|
- Inode 10 with valid refcount
|
|
|
|
Now when recovery code runs, it needs "enforce" this state on the file
|
|
system. This is what guarantees idempotence of fast commit replay.
|
|
|
|
Let's take an example of a procedure that is not idempotent and see how fast
|
|
commits make it idempotent. Consider following sequence of operations:
|
|
|
|
1) rm A
|
|
2) mv B A
|
|
3) read A
|
|
|
|
If we store this sequence of operations as is then the replay is not idempotent.
|
|
Let's say while in replay, we crash after (2). During the second replay,
|
|
file A (which was actually created as a result of "mv B A" operation) would get
|
|
deleted. Thus, file named A would be absent when we try to read A. So, this
|
|
sequence of operations is not idempotent. However, as mentioned above, instead
|
|
of storing the procedure fast commits store the outcome of each procedure. Thus
|
|
the fast commit log for above procedure would be as follows:
|
|
|
|
(Let's assume dirent A was linked to inode 10 and dirent B was linked to
|
|
inode 11 before the replay)
|
|
|
|
1) Unlink A
|
|
2) Link A to inode 11
|
|
3) Unlink B
|
|
4) Inode 11
|
|
|
|
If we crash after (3) we will have file A linked to inode 11. During the second
|
|
replay, we will remove file A (inode 11). But we will create it back and make
|
|
it point to inode 11. We won't find B, so we'll just skip that step. At this
|
|
point, the refcount for inode 11 is not reliable, but that gets fixed by the
|
|
replay of last inode 11 tag. Thus, by converting a non-idempotent procedure
|
|
into a series of idempotent outcomes, fast commits ensured idempotence during
|
|
the replay.
|
|
|
|
Journal Checkpoint
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Checkpointing the journal ensures all transactions and their associated buffers
|
|
are submitted to the disk. In-progress transactions are waited upon and included
|
|
in the checkpoint. Checkpointing is used internally during critical updates to
|
|
the filesystem including journal recovery, filesystem resizing, and freeing of
|
|
the journal_t structure.
|
|
|
|
A journal checkpoint can be triggered from userspace via the ioctl
|
|
EXT4_IOC_CHECKPOINT. This ioctl takes a single, u64 argument for flags.
|
|
Currently, three flags are supported. First, EXT4_IOC_CHECKPOINT_FLAG_DRY_RUN
|
|
can be used to verify input to the ioctl. It returns error if there is any
|
|
invalid input, otherwise it returns success without performing
|
|
any checkpointing. This can be used to check whether the ioctl exists on a
|
|
system and to verify there are no issues with arguments or flags. The
|
|
other two flags are EXT4_IOC_CHECKPOINT_FLAG_DISCARD and
|
|
EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT. These flags cause the journal blocks to be
|
|
discarded or zero-filled, respectively, after the journal checkpoint is
|
|
complete. EXT4_IOC_CHECKPOINT_FLAG_DISCARD and EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT
|
|
cannot both be set. The ioctl may be useful when snapshotting a system or for
|
|
complying with content deletion SLOs.
|