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Re: Contiguous file sequences

To: Daire Byrne <daire.byrne@xxxxxxxxx>
Subject: Re: Contiguous file sequences
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Sun, 26 Sep 2010 23:08:57 -0500
Cc: xfs@xxxxxxxxxxx
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Daire Byrne wrote:
> Eric,
> 
> On Wed, Sep 22, 2010 at 9:10 PM, Eric Sandeen <sandeen@xxxxxxxxxxx> wrote:
>> Daire Byrne wrote:
>>> Hi,
>>>
>>> I have been trying to figure out how to lay down a file sequence (e.g.
>>> images) such that they are guaranteed to always be contiguous on disk
>>> (i.e. no block gaps between them).
>> There's no mechanism to guarantee that.
>>
>> Why is this the goal, what are you trying to achieve?
> 
> I am essentially trying to play back a large frame sequence and trying
> to minimise seeks as it can lead to sporadic slowdowns on a SATA based
> RAID.

Ok - and you've really seen allocation patterns that cause the playback
to slow down?  xfs_bmap information for a few sequential files that were
this far off would be interesting to see.

Are you certain that it's seekiness causing the problem?  A great way
to visualize it would be to use the seekwatcher application while you
run a problematic file sequence.

...

>> You can't specify a starting block for any given file I'm afraid.
> 
> Somebody pointed me at this which looks fairly promising:
> 
>   http://oss.sgi.com/archives/xfs/2006-07/msg01005.html

Yeah, that never got merged, but I think it still could be.

It's only half your battle though, you need to find that contiguous
space first, then specify the start block for it with the interface
above.

> I'm still trying to get my head around how I would actually write a
> userspace app/script to use it but I think it should allow me to do
> what I want. It would be good if I could script it through xfs_io. I'd
> really like a script where I could point it at a directory and it
> would do something like:
> 
>   1. count total space used by file sequence
>   2. find start block for that much contiguous space on disk (or as
> much of it as possible)
>   3. allocate the files using the start block one after another on disk
> 
>>> Another option might be to create a single contiguous large file,
>>> concatenate all the images into it and then split it up on disk using
>>> offsets but I don't think such a thing is even possible? I always know
>>> the image sequence size beforehand, all images are exactly the same
>>> size and I can control/freeze the filesystem access if needed.
>>>
>>> Anybody got any suggestions? It *seems* like something that should be
>>> possible and would be useful.
>> This would be pretty low-level control of the allocator by userspace.
>>
>> I'll just go back and ask what problem you're trying to solve?  There
>> may be a better (i.e. currently existing) solution.
> 
> The "realtime" option is sometimes suggested as a way to do sequence
> streaming but I'd really rather avoid that. It seems to me like the
> option to allocate a sequence of files end on end in a known chunk of
> contiguous space is something that would be useful in the normal
> operating mode.

It would be, but it's not there now.  Also, without some more complexity
it'd still probably end up being a best effort rather than a guarantee,
but some hints from userspace might be better than nothing.

-ERic

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