Re: [PATCH] tests: fix false du failure on newer XFS

[Pádraig Brady]

On 09/16/2014 12:54 PM, Rasmus Borup Hansen wrote:
> On 16 Sep 2014, at 13:18, Bernhard Voelker wrote:
> 
>> On 09/16/2014 12:48 PM, Bernhard Voelker wrote:
>>> So XFS really allocates 4G for a 2G file. ;-/
>>> I consider this a bug in XFS - maybe on {open,}SUSE only? [...]
>>
>> I filed a bug report for this, too:
>> https://bugzilla.novell.com/show_bug.cgi?id=896890
>>
>> Have a nice day,
>> Berny
>>
>>
> 
> I haven't been following this thread closely, but I'd guess this is related 
> to speculative preallocation. Check the last five questions in the XFS FAQ: 
> http://xfs.org/index.php/XFS_FAQ

Yes. There are two particular aspects of XFS speculative preallocation that are 
triggering this:

1. "Applications that repeatedly trigger preallocation and reclaim cycles 
[after file close] can cause fragmentation.
   Therefore, this pattern is detected and causes the preallocation to persist 
beyond the lifecycle of the file descriptor."

2. "Preallocation sizes grow as files grow larger."

Berhard's patch avoids 1 by only doing a single close (reclaim cycle).
Bernhard it would be worth mentioning the above two points in the
commit message as they're not obvious, and then the patch is fine to push.

thanks!
Pádraig.