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    SubjectRe: [RFC] new ->perform_write fop
    On Sat 22-05-10 10:27:59, Dave Chinner wrote:
    > On Fri, May 21, 2010 at 08:58:46PM +0200, Jan Kara wrote:
    > > On Fri 21-05-10 09:05:24, Dave Chinner wrote:
    > > > On Thu, May 20, 2010 at 10:12:32PM +0200, Jan Kara wrote:
    > > > > b) E.g. ext4 can do even without hole punching. It can allocate extent
    > > > > as 'unwritten' and when something during the write fails, it just
    > > > > leaves the extent allocated and the 'unwritten' flag makes sure that
    > > > > any read will see zeros. I suppose that other filesystems that care
    > > > > about multipage writes are able to do similar things (e.g. btrfs can
    > > > > do the same as far as I remember, I'm not sure about gfs2).
    > > >
    > > > Allocating multipage writes as unwritten extents turns off delayed
    > > > allocation and hence we'd lose all the benefits that this gives...
    > > Ah, sorry. That was a short-circuit in my brain. But when we do delayed
    > > I don't see why we should actually do any hole punching... The write needs
    > > to:
    > > a) reserve enough blocks for the write - I don't know about other
    > > filesystems but for ext4 this means just incrementing a counter.
    > > b) copy data page by page
    > > c) release part of reservation (i.e. decrement counter) if we actually
    > > copied less than we originally thought.
    > >
    > > Am I missing something?
    > Possibly. Delayed allocation is made up of two parts - space
    > reservation and recording the regions of delayed allocation in an
    > extent tree, page/bufferhead state or both.
    Yes. Ext4 records the info about delayed allocation only in buffer

    > In XFS, these two steps happen in the same get_blocks call, but the
    > result of that is we have to truncate/punch delayed allocate extents
    > out just like normal extents if we are not going to use them. Hence
    > a reserve/allocate interface allows us to split the operation -
    > reserve ensures we have space for the delayed allocation, allocate
    > inserts the delayed extents into the inode extent tree for later
    > real allocation during writeback. Hence the unreserve call can
    > simply be accounting - it has no requirement to punch out delayed
    > extents that may have already been allocated, just do work on
    > counters.
    > btrfs already has this split design - it reserves space, does the
    > copy, then marks the extent ranges as delalloc once the copy has
    > succeeded, otherwise it simply unreserves the unused space.
    > Once again, I don't know if ext4 does this internal delayed
    > allocation extent tracking or whether it just uses page state to
    > track those extents, but it would probably still have to use the
    > allocate call to mark all the pages/bufferheads as delalloc so
    > that uneserve didn't have to do any extra work.
    Yes, exactly. I just wanted to point out that AFAICS ext4 can implement
    proper error recovery without a need for 'punch' operation. So after all
    Nick's copy page-by-page should be plausible at least for ext4.

    Jan Kara <>
    SUSE Labs, CR

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