Re: XFS fallocate implementation incorrectly reports ENOSPC

[Chris Dunlop <chris@onthe.net.au>]

On Thu, Aug 26, 2021 at 10:05:00AM -0500, Eric Sandeen wrote:
> On 8/25/21 9:06 PM, Chris Dunlop wrote:
>>
>> fallocate -l 1GB image.img
>> mkfs.xfs -f image.img
>> mkdir mnt
>> mount -o loop ./image.img mnt
>> fallocate -o 0 -l 700mb mnt/image.img
>> fallocate -o 0 -l 700mb mnt/image.img
>>
>> Why does the second fallocate fail with ENOSPC, and is that considered an XFS bug?
>
> Interesting.  Off the top of my head, I assume that xfs is not looking at
> current file space usage when deciding how much is needed to satisfy the
> fallocate request.  While filesystems can return ENOSPC at any time for
> any reason, this does seem a bit suboptimal.

Yes, I would have thought the second fallocate should be a noop.

>> Background: I'm chasing a mysterious ENOSPC error on an XFS filesystem 
>> with way more space than the app should be asking for. There are no 
>> quotas on the fs. Unfortunately it's a third party app and I can't tell 
>> what sequence is producing the error, but this fallocate issue is a 
>> possibility.
>
> Presumably you've tried stracing it and looking for ENOSPC returns from
> syscalls?

That would be an obvious approach. Unfortunately it's not that easy. The 
problem is associated with one specific client which is out of my control 
so I can't experiment in a controlled environment. The app runs for 
several hours in multiple phases, each with multiple threads, and the 
problem typically occurs in the early hours of the morning after several 
hours of running, so attaching to the correct instance is fraught, and the 
strace output will be voluminous.

Cheers,

Chris