Re: Performance of ext4
From: Holger Kiehl
Date: Fri Jun 20 2008 - 04:33:48 EST
On Thu, 19 Jun 2008, Theodore Tso wrote:
On Thu, Jun 19, 2008 at 11:41:17AM -0500, Eric Sandeen wrote:
It might be worth runninga "simple" fsx under your kernel too; last time
I tested fsx it was still happy and it exercises fs ops (including
truncate) at random...
From what Holger described, it's doubtful that the bug is in the
truncate operation.
Correct, the benchmark just copies, moves, hardlinks and deletes a lot
of small files. It also overwrites existing files but not at the same
scale it does the other operations.
It sounds like i_size is actually dropping in
size at some pointer long after the file was written. If I had to
guess the value in the inode cache is correct; and perhaps so is the
value on the journal. But somehow, the wrong value is getting written
to disk (remember the jbd layer can keep up to three different
versions of filesystem metadata in memory, because most of the time we
don't block modifications to the filesystem while we are in the middle
of writing a previous commit to disk). So depending on whether the
inode gets redirtied or not, the inconsistency could self-heal, and if
the inode never gets pushed out of memory due to memory pressure, the
problem might not be noticed until the system reboots or the
filesystem is unmounted.
I always had the feeling that waiting a day or unmounting caused a lot
more truncation. On my system at home for example I mounted the test
filesystem again and saw that files where truncated and I am pretty sure
that when I looked at those files during and shortly after the test they
where still complete. But I will recheck and do test as you suggested.
What I find strange is that the missing parts of the file are not for
example exactly 512 or 1024 or 4096 bytes it is mostly some odd number
of bytes.
Holger
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