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? > Currently if I write a sequence to > disk things like "filestreams" help keep everything in the same AG and > the allocation algorithm seems to prefer to try and place files next > to eachother but without the filesystem knowing the total size of the > sequence there are always likely to be gaps in the blocks where > existing data has been written. preallocation of each image before writing would help make it more likely that each image is itself contiguous (but again this is not -guaranteed-) > So even if the first file is written > completely contiguously to disk there is no way to guarantee that > there is contiguous free space after it to write the rest of the > images. > > What I really want is to be able to find and reserve enough space for > the entire sequence and then write the files into that big contiguous > range. I tried to do this with xfs_io hoping that the allocator would > just know what I wanted and do the right thing (ever the optimist...). :) > So something like this: > > # find and reserve a big chunk to fit all my files in > xfs_io -f -c "resvsp 0 136314880" -c "bmap -v" $DIR/test.0 > > # now shrink it keeping the start block > xfs_io -f -c "freesp 13631488 0" -c "bmap -v" $DIR/test.0 > > # now write a bunch of files and hope they continue from test.0 on disk > dd if=/dev/zero of=$DIR/test.0 bs=1M count=13 conv=nocreat,notrunc > for x in `seq 1 4`; do > dd if=/dev/zero of=$DIR/test.$x bs=1M count=13 conv=notrunc > done > > But a new allocation is made for the first new file in the sequence > elsewhere on disk and I don't know how to get it to use the large > chunk of free contiguous space after the "test.0" file instead. You can't specify a starting block for any given file I'm afraid. > 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. -Eric > Daire > > _______________________________________________ > xfs mailing list > xfs@xxxxxxxxxxx > http://oss.sgi.com/mailman/listinfo/xfs > _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
