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    SubjectRe: [patch] delayed disk block allocation
    Date
    On March 4, 2002 08:53 am, Andrew Morton wrote:
    > Andreas Dilger wrote:
    > >
    > > Actually, there are a whole bunch of performance issues with 1kB block
    > > ext2 filesystems.
    >
    > I don't see any new problems with file tails here. They're catered for
    > OK. What I have not catered for is file holes. With the delalloc
    > patches, whole pages are always written out (except for at eof). So
    > if your file has lots of very small non-holes in it, these will become
    > larger non-holes.
    >
    > If we're serious about 64k PAGE_CACHE_SIZE then this becomes more of
    > a problem. In the worst case, a file which used to consist of
    >
    > 4k block | (1 meg - 4k) hole | 4k block | (1 meg - 4k) hole | ...
    >
    > will become:
    >
    > 64k block | (1 meg - 64k) hole | 64k block | (1 meg - 64k hole) | ...
    >
    > Which is a *lot* more disk space. It'll happen right now, if the
    > file is written via mmap. But with no-buffer-head delayed allocate,
    > it'll happen with write(2) as well.
    >
    > Is such space wastage on sparse files a showstopper? Maybe,
    > but probably not if there is always at least one filesystem
    > which handles this scenario well, because it _is_ a specialised
    > scenario.

    I guess 4K PAGE_CACHE_SIZE will serve us well for another couple of years,
    and in that time I hope to produce a patch that generalizes the notion of
    page size so we can use the size that works best for each address_space,
    i.e., the same as the filesystem blocksize.

    By the way, have you ever seen a sparse 1K blocksize file? I haven't, I
    wouldn't get too worked up about treating the holes a little less than
    optimally.

    --
    Daniel
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