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spurious -ENOSPC on XFS

To: xfs@xxxxxxxxxxx
Subject: spurious -ENOSPC on XFS
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date: Mon, 12 Jan 2009 06:14:36 -0500 (EST)
Cc: linux-kernel@xxxxxxxxxxxxxxx

I discovered a bug in XFS in delayed allocation.

When you take a small partition (52MB in my case) and copy many small 
files on it (source code) that barely fits there, you get -ENOSPC. Then 
sync the partition, some free space pops up, click "retry" in MC an the 
copy continues. They you get again -ENOSPC, you must sync, click "retry" 
and go on. And so on few times until the source code finally fits on the 
XFS partition.

This misbehavior is apparently caused by delayed allocation, delayed 
allocation does not exactly know how much space will be occupied by data, 
so it makes some upper bound guess. Because free space count is only a 
guess, not the actual data being consumed, XFS should not return -ENOSPC 
on behalf of it. When the free space overflows, XFS should sync itself, 
retry allocation and only return -ENOSPC if it fails the second time, 
after the sync.


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