spurious -ENOSPC on XFS

Mikulas Patocka
2009-01-12T11:14:53Z

Hi

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.

Mikulas