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[BUG]
There is a small workload which will always fail with recent kernel:
(A simplified version from btrfs/125 test case)
mkfs.btrfs -f -m raid5 -d raid5 -b 1G $dev1 $dev2 $dev3
mount $dev1 $mnt
xfs_io -f -c "pwrite -S 0xee 0 1M" $mnt/file1
sync
umount $mnt
btrfs dev scan -u $dev3
mount -o degraded $dev1 $mnt
xfs_io -f -c "pwrite -S 0xff 0 128M" $mnt/file2
umount $mnt
btrfs dev scan
mount $dev1 $mnt
btrfs balance start --full-balance $mnt
umount $mnt
The failure is always failed to read some tree blocks:
BTRFS info (device dm-4): relocating block group 217710592 flags data|raid5
BTRFS error (device dm-4): parent transid verify failed on 38993920 wanted 9 found 7
BTRFS error (device dm-4): parent transid verify failed on 38993920 wanted 9 found 7
...
[CAUSE]
With the recently added debug output, we can see all RAID56 operations
related to full stripe 38928384:
56.1183: raid56_read_partial: full_stripe=38928384 devid=2 type=DATA1 offset=0 opf=0x0 physical=9502720 len=65536
56.1185: raid56_read_partial: full_stripe=38928384 devid=3 type=DATA2 offset=16384 opf=0x0 physical=9519104 len=16384
56.1185: raid56_read_partial: full_stripe=38928384 devid=3 type=DATA2 offset=49152 opf=0x0 physical=9551872 len=16384
56.1187: raid56_write_stripe: full_stripe=38928384 devid=3 type=DATA2 offset=0 opf=0x1 physical=9502720 len=16384
56.1188: raid56_write_stripe: full_stripe=38928384 devid=3 type=DATA2 offset=32768 opf=0x1 physical=9535488 len=16384
56.1188: raid56_write_stripe: full_stripe=38928384 devid=1 type=PQ1 offset=0 opf=0x1 physical=30474240 len=16384
56.1189: raid56_write_stripe: full_stripe=38928384 devid=1 type=PQ1 offset=32768 opf=0x1 physical=30507008 len=16384
56.1218: raid56_write_stripe: full_stripe=38928384 devid=3 type=DATA2 offset=49152 opf=0x1 physical=9551872 len=16384
56.1219: raid56_write_stripe: full_stripe=38928384 devid=1 type=PQ1 offset=49152 opf=0x1 physical=30523392 len=16384
56.2721: raid56_parity_recover: full stripe=38928384 eb=39010304 mirror=2
56.2723: raid56_parity_recover: full stripe=38928384 eb=39010304 mirror=2
56.2724: raid56_parity_recover: full stripe=38928384 eb=39010304 mirror=2
Before we enter raid56_parity_recover(), we have triggered some metadata
write for the full stripe 38928384, this leads to us to read all the
sectors from disk.
Furthermore, btrfs raid56 write will cache its calculated P/Q sectors to
avoid unnecessary read.
This means, for that full stripe, after any partial write, we will have
stale data, along with P/Q calculated using that stale data.
Thankfully due to patch "btrfs: only write the sectors in the vertical stripe
which has data stripes" we haven't submitted all the corrupted P/Q to disk.
When we really need to recover certain range, aka in
raid56_parity_recover(), we will use the cached rbio, along with its
cached sectors (the full stripe is all cached).
This explains why we have no event raid56_scrub_read_recover()
triggered.
Since we have the cached P/Q which is calculated using the stale data,
the recovered one will just be stale.
In our particular test case, it will always return the same incorrect
metadata, thus causing the same error message "parent transid verify
failed on 39010304 wanted 9 found 7" again and again.
[BTRFS DESTRUCTIVE RMW PROBLEM]
Test case btrfs/125 (and above workload) always has its trouble with
the destructive read-modify-write (RMW) cycle:
0 32K 64K
Data1: | Good | Good |
Data2: | Bad | Bad |
Parity: | Good | Good |
In above case, if we trigger any write into Data1, we will use the bad
data in Data2 to re-generate parity, killing the only chance to recovery
Data2, thus Data2 is lost forever.
This destructive RMW cycle is not specific to btrfs RAID56, but there
are some btrfs specific behaviors making the case even worse:
- Btrfs will cache sectors for unrelated vertical stripes.
In above example, if we're only writing into 0~32K range, btrfs will
still read data range (32K ~ 64K) of Data1, and (64K~128K) of Data2.
This behavior is to cache sectors for later update.
Incidentally commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.