On 2023/12/4 20:11, Jan Kara wrote:
Hello!
Thank you for your reply!
On Sat 02-12-23 17:14:30, Baokun Li wrote:
Recently, while running some pressure tests on MYSQL, noticed that
occasionally a "corrupted data in log event" error would be reported.
After analyzing the error, I found that extending DIO write and buffered
read were competing, resulting in some zero-filled page end being read.
Since ext4 buffered read doesn't hold an inode lock, and there is no
field in the page to indicate the valid data size, it seems to me that
it is impossible to solve this problem perfectly without changing these
two things.
Yes, combining buffered reads with direct IO writes is a recipe for
problems and pretty much in the "don't do it" territory. So honestly I'd
consider this a MYSQL bug. Were you able to identify why does MYSQL use
buffered read in this case? It is just something specific to the test
you're doing?
The problem is with a one-master-twoslave MYSQL database with three
physical machines, and using sysbench pressure testing on each of the
three machines, the problem occurs about once every two to three hours.
The problem is with the relay log file, and when the problem occurs, the
middle dozens of bytes of the file are read as all zeros, while the data on
disk is not. This is a journal-like file where a write process gets the
data from
the master node and writes it locally, and another replay process reads the
file and performs the replay operation accordingly (some SQL statements).
The problem is that when replaying, it finds that the data read is
corrupted,
not valid SQL data, while the data on disk is normal.
It's not confirmed that buffered reads vs direct IO writes is actually
causing
this issue, but this is the only scenario that we can reproduce with our
local
simplified scripts. Also, after merging in patch 1, the MYSQL pressure test
scenario has now been tested for 5 days and has not been reproduced.
I'll double-check the problem scenario, although buffered reads with
buffered
writes doesn't seem to have this problem.
In this series, the first patch reads the inode size twice, and takes the
smaller of the two values as the copyout limit to avoid copying data that
was not actually read (0-padding) into the user buffer and causing data
corruption. This greatly reduces the probability of problems under 4k
page. However, the problem is still easily triggered under 64k page.
The second patch waits for the existing dio write to complete and
invalidate the stale page cache before performing a new buffered read
in ext4, avoiding data corruption by copying the stale page cache to
the user buffer. This makes it much less likely that the problem will
be triggered in a 64k page.
Do we have a plan to add a lock to the ext4 buffered read or a field in
the page that indicates the size of the valid data in the page? Or does
anyone have a better idea?
No, there are no plans to address this AFAIK. Because such locking will
slow down all the well behaved applications to fix a corner case for
application doing unsupported things. Sure we must not crash the kernel,
corrupt the filesystem or leak sensitive (e.g. uninitialized) data if app
combines buffered and direct IO but returning zeros instead of valid data
is in my opinion fully within the range of acceptable behavior for such
case.
Honza
I also feel that a scenario like buffered reads + DIO writes is strange. But
theoretically when read doesn't return an error, the data read shouldn't
be wrong.
And I tested that xfs guarantees data consistency in this scenario,
which is why I
thought it might be buggy.
Thanks!
--
With Best Regards,
Baokun Li
.
