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 _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
