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Re: Is persistant preallocation possible?

To: Nathan Scott <nathans@xxxxxxxxxx>
Subject: Re: Is persistant preallocation possible?
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Mon, 24 May 2010 22:19:52 -0500
Cc: Jeff Zheng <Jeff.Zheng@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <1374176788.272721274754374474.JavaMail.root@xxxxxxxxxxxxxxxxxx>
References: <1374176788.272721274754374474.JavaMail.root@xxxxxxxxxxxxxxxxxx>
User-agent: Thunderbird 2.0.0.24 (Macintosh/20100228)
Nathan Scott wrote:
> ----- "Jeff Zheng" <Jeff.Zheng@xxxxxxxxxx> 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

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