[Bug 78651] Write performance of ext4 degrades linearly as volume fills

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=78651

Theodore Tso <tytso@mit.edu> changed:

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                 CC|                            |tytso@mit.edu

--- Comment #5 from Theodore Tso <tytso@mit.edu> ---
Delayed allocation certainly works with or without the journal.  But disabling
delayed allocation will _significantly_ impact performance.   There's certainly
no surprise there.   Delalloc is one of the reasons why ext4 is significantly
more performant than ext3, and the mode that we use in Google is ext4 with
delalloc in no journal mode.

In answer to your other questions, no the es_shrink_enter and es_shrink_exit
calls are not balanced.  In particular, the ext4_es_shrink_enter tracepoint
gets called from two different functions (which is a bad thing; recent shrinker
infrastructure changes added a s_es_shrinker_.count_objects() callback, and the
person converted the ext4 shirnker over to the new setup duplicated the
tracepoint instead of creating a new one.   Also, in ext4_es_scan(), if
nr_to_scan is zero, we don't end up calling the ext4_es_shrinker_exit
tracepoint.

Some other things to try.  (1)  Try collecting copies of /proc/meminfo and
/proc/slabinfo every 10% of the dump process or so.  That might be useful.  (2)
Try reformatting with a much larger journal, and see if that makes a
difference.  I doubt it will, but it's worth a try.   (3)   Either using
/sys/kernel/debug, or the ftrace command (don't use perf; since we need the
data associated with the tracepoints, not just the count), enable the
jbd2_checkpoint and jbd2_run_stats and collect the tracepoint data during the
run.

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