linux-ext4 - Bigalloc ENOSPC woes

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Date:	Mon, 30 Jan 2012 15:19:05 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Bigalloc ENOSPC woes

I got nervous when I tried this on an -O bigalloc -C 65536 fs:

# echo foo > blah; du -hc blah; sync; du -hc blah
4.0K blah
4.0K total
64K blah
64K total

Seems that du should never report less than the cluster size, should it?

And that made me wonder about ENOSPC handling - are we tracing delalloc allocations correctly?

>From running xfstest 204 on a similarly-created fs, it really seems that we are not.

A manual test of creating 4-byte files until ENOSPC (which was really when I ran out of inodes) and then issuing a sync led to a storm of kernel messages about allocation failures:

[  482.154538] EXT4-fs (sdb6): delayed block allocation failed for inode 1664 at logical offset 0 with max blocks 1 with error -28
[  482.154540] EXT4-fs (sdb6): This should not happen!! Data will be lost

<ad infinitum, or at least 600 or so in my case>

and an end result of a bunch of 4 byte long files with no extents.  Almost like files full of NULLs.  Careful - that is hard to live down.  :)

Ted, are you aware of those issues in bigalloc?

-Eric
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