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86 lines
4.3 KiB
ReStructuredText
86 lines
4.3 KiB
ReStructuredText
Tree checker
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============
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Tree checker is a feature that verifies metadata blocks before write or after
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read from the devices. The b-tree nodes contain several items describing the
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filesystem structures and to some degree can be verified for consistency or
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validity. This is an additional check to the checksums that only verify the
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overall block status while the tree checker tries to validate and cross
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reference the logical structure. This takes a slight performance hit but is
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comparable to calculating the checksum and has no noticeable impact while it
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does catch all sorts of errors.
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There are two occasions when the checks are done:
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Pre-write checks
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----------------
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When metadata blocks are in memory and about to be written to the permanent
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storage, the checks are performed, before the checksums are calculated. This
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can catch random corruptions of the blocks (or pages) either caused by bugs or
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by other parts of the system or hardware errors (namely faulty RAM).
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Once a block does not pass the checks, the filesystem refuses to write more data
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and turns itself to read-only mode to prevent further damage. At this point some
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the recent metadata updates are held *only* in memory so it's best to not panic
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and try to remember what files could be affected and copy them elsewhere. Once
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the filesystem gets unmounted, the most recent changes are unfortunately lost.
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The filesystem that is stored on the device is still consistent and should mount
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fine.
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A message may look like:
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.. code-block::
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[ 1716.823895] BTRFS critical (device vdb): corrupt leaf: root=18446744073709551607 block=38092800 slot=0, invalid key objectid: has 1 expect 6 or [256, 18446744073709551360] or 18446744073709551604
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[ 1716.829499] BTRFS info (device vdb): leaf 38092800 gen 19 total ptrs 4 free space 15851 owner 18446744073709551607
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[ 1716.832891] BTRFS info (device vdb): refs 3 lock (w:0 r:0 bw:0 br:0 sw:0 sr:0) lock_owner 0 current 1506
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[ 1716.836054] item 0 key (1 1 0) itemoff 16123 itemsize 160
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[ 1716.837993] inode generation 1 size 0 mode 100600
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[ 1716.839760] item 1 key (256 1 0) itemoff 15963 itemsize 160
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[ 1716.841742] inode generation 4 size 0 mode 40755
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[ 1716.843393] item 2 key (256 12 256) itemoff 15951 itemsize 12
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[ 1716.845320] item 3 key (18446744073709551611 48 1) itemoff 15951 itemsize 0
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[ 1716.847505] BTRFS error (device vdb): block=38092800 write time tree block corruption detected
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The line(s) before the *write time tree block corruption detected* message is
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specific to the found error.
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Post-read checks
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----------------
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Metadata blocks get verified right after they're read from devices and the
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checksum is found to be valid. This protects against changes to the metadata
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that could possibly also update the checksum, less likely to happen accidentally
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but rather due to intentional corruption or fuzzing.
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.. code-block::
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[ 4823.612832] BTRFS critical (device vdb): corrupt leaf: root=7 block=30474240 slot=0, invalid nritems, have 0 should not be 0 for non-root leaf
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[ 4823.616798] BTRFS error (device vdb): block=30474240 read time tree block corruption detected
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The checks
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----------
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As implemented right now, the metadata consistency is limited to one b-tree node
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and what items are stored there, i.e. there's no extensive or broad check done
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e.g. against other data structures in other b-tree nodes. This still provides
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enough opportunities to verify consistency of individual items, besides verifying
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general validity of the items like the length or offset. The b-tree items are
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also coupled with a key so proper key ordering is also part of the check and can
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reveal random bitflips in the sequence (this has been the most successful
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detector of faulty RAM).
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The capabilities of tree checker have been improved over time and it's possible
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that a filesystem created on an older kernel may trigger warnings or fail some
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checks on a new one.
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Reporting problems
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------------------
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In many cases the bug is caused by hardware and cannot be automatically fixed
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by *btrfs check --repair*, so do not try that without being advised to. Even if
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the error is unfixable it's useful to report it, either to validate the cause
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but also to give more ideas how to improve the tree checker. Please consider
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reporting it to the mailing list *linux-btrfs@vger.kernel.org*.
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