Is persistant preallocation possible?

[Eric Sandeen]

Nathan Scott wrote:
> ----- "Jeff Zheng" <Jeff.Zheng at endace.com> wrote:
> 
>> Hi,
>>
>> We are trying to control where the files are being allocated on the
>> disks, and we tried xfs preallocation. What we found is that each time
>> the file being reopened the location of the pre-allocated space could
>> change (not all the time). Is there a way to do persistent
>> preallocation in xfs, like the same file will always be on the same
>> location on the disk given that the requested size doesn’t change?
>>
> 
> Sounds like you are truncating the file (on open?), and hence discarding
> the preallocation.  XFS preallocation is persistent already, so I think
> something you are doing must be discarding those preallocated extents.
> 
> Could you post a test program, or sequence of xfs_io commands, that would
> demonstrate the problem?
> 
> cheers.
> 

... maybe you mean that truncating / pre-allocating the same space repeatedly
on the same filename ends up with blocks mapped in different locations...?
That's entirely expected... nothing about the preallocation interface lets
you "control *where* files are being allocated" as you said above...

-Eric