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    SubjectRe: [RFC] delayed allocation for ext4
    On Fri, Dec 22, 2006 at 11:20:08PM +0300, Alex Tomas wrote:
    > Good day,
    > probably the previous set of patches (including mballoc/lg)
    > is too large. so, I reworked delayed allocation a bit so
    > that it can be used on top of regular balloc, though it
    > still can be used with extents-enabled files only.
    > this time series contains just 3 patches:
    > - booked-page-flag.patch
    > adds PG_booked bit to page->flags. it's used in delayed
    > allocation to mark space is already reserved for page
    > (including possible metadata)

    So that mean's we'll have 2 separate mechanisms for marking
    pages as delalloc. XFS uses the BH_delay flag to indicate
    that a buffer (block) attached to the page is using delalloc.

    FWIW, how does this mechanism deal with block size < page size?
    Don't you have to track delalloc on a block basis rather than
    a page basis?

    > - ext4-block-reservation.patch
    > this is scalable free space management. every time we
    > delay allocation of some page, a space (including metadata)
    > should be reserved
    > - ext4-delayed-allocation.patch
    > delayed allocation itself, enabled by "delalloc" mount option.
    > extents support is also required. currently it works only
    > with blocksize=pagesize.

    Ah, that's why you can get away with a page flag - you've ignored
    the partial page delay state problem. Any plans to use the
    existing method in the future so we will be able to use ext4 delalloc
    on machines with a page size larger than 4k?


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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