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    SubjectRe: [00/17] Large Blocksize Support V3
    On Wednesday 25 April 2007 01:21, wrote:
    > V2->V3
    > - More restructuring
    > - It actually works!
    > - Add XFS support
    > - Fix up UP support
    > - Work out the direct I/O issues
    > - Add CONFIG_LARGE_BLOCKSIZE. Off by default which makes the inlines revert
    > back to constants. Disabled for 32bit and HIGHMEM configurations.
    > This also allows a gradual migration to the new page cache
    > inline functions. LARGE_BLOCKSIZE capabilities can be
    > added gradually and if there is a problem then we can disable
    > a subsystem.
    > V1->V2
    > - Some ext2 support
    > - Some block layer, fs layer support etc.
    > - Better page cache macros
    > - Use macros to clean up code.
    > This patchset modifies the Linux kernel so that larger block sizes than
    > page size can be supported. Larger block sizes are handled by using
    > compound pages of an arbitrary order for the page cache instead of
    > single pages with order 0.
    > Rationales:
    > 1. We have problems supporting devices with a higher blocksize than
    > page size. This is for example important to support CD and DVDs that
    > can only read and write 32k or 64k blocks. We currently have a shim
    > layer in there to deal with this situation which limits the speed
    > of I/O. The developers are currently looking for ways to completely
    > bypass the page cache because of this deficiency.
    > 2. 32/64k blocksize is also used in flash devices. Same issues.
    > 3. Future harddisks will support bigger block sizes that Linux cannot
    > support since we are limited to PAGE_SIZE. Ok the on board cache
    > may buffer this for us but what is the point of handling smaller
    > page sizes than what the drive supports?
    > 4. Reduce fsck times. Larger block sizes mean faster file system checking.
    > 5. Performance. If we look at IA64 vs. x86_64 then it seems that the
    > faster interrupt handling on x86_64 compensate for the speed loss due to
    > a smaller page size (4k vs 16k on IA64). Supporting larger block sizes
    > sizes on all allows a significant reduction in I/O overhead and
    > increases the size of I/O that can be performed by hardware in a single
    > request since the number of scatter gather entries are typically limited
    > for one request. This is going to become increasingly important to support
    > the ever growing memory sizes since we may have to handle excessively large
    > amounts of 4k requests for data sizes that may become common soon. For
    > example to write a 1 terabyte file the kernel would have to handle 256
    > million 4k chunks.
    > 6. Cross arch compatibility: It is currently not possible to mount
    > an 16k blocksize ext2 filesystem created on IA64 on an x86_64 system.
    > With this patch this becoems possible.
    > The support here is currently only for buffered I/O. Modifications for
    > three filesystems are included:
    > A. XFS
    > B. Ext2
    > C. ramfs
    > Unsupported
    > - Mmapping blocks larger than page size
    > Issues:
    > - There are numerous places where the kernel can no longer assume that the
    > page cache consists of PAGE_SIZE pages that have not been fixed yet.
    > - Defrag warning: The patch set can fragment memory very fast.
    > It is likely that Mel Gorman's anti-frag patches and some more
    > work by him on defragmentation may be needed if one wants to use
    > super sized pages.
    > If you run a 2.6.21 kernel with this patch and start a kernel compile
    > on a 4k volume with a concurrent copy operation to a 64k volume on
    > a system with only 1 Gig then you will go boom (ummm no ... OOM) fast.
    > How well Mel's antifrag/defrag methods address this issue still has to
    > be seen.
    > Future:
    > - Mmap support could be done in a way that makes the mmap page size
    > independent from the page cache order. It is okay to map a 4k section
    > of a larger page cache page via a pte. 4k mmap semantics can be
    > completely preserved even for larger page sizes.
    > - Maybe people could perform benchmarks to see how much of a difference
    > there is between 4k size I/O and 64k? Andrew surely would like to know.
    > - If there is a chance for inclusion then I will diff this against mm,
    > do a complete scan over the kernel to find all page cache == PAGE_SIZE
    > assumptions and then try to get it upstream for 2.6.23.
    > How to make this work:
    > 1. Apply this patchset to 2.6.21-rc7
    > 2. Configure LARGE_BLOCKSIZE Support
    > 3. compile kernel
    > --

    I really like the idea of block size > page size

    I just want to suggest mine ideas about how to implement it (I can't do that
    since it is too compicated for me and I don't have time now)

    I visualized this like that:

    For size <= 4K , page still holds a 1 or more blocks.

    For size > 4K:

    A page holds a fraction of block, and its ->buffer_head also holds info about
    that fraction.

    But that ->bh contains a ->next pointer (or a list_head) that combines
    all fractions of block in single one.

    This will minimize changes in fs code.

    Today fs blindly uses bh retured by block code.
    A modifed filesystem will also read from linked ->next bhs to get all parts of

    For blocksizes <= 4k that ->next will be null indicating that that bh
    contatins whole block.

    Then inplementation of block address_space opertions should not change a lot

    They will be just aware that that page can be linked with other pages of same
    block and do that right thing.

    For example:

    ->readpage will not only read _that_ page but also will read all sibling pages
    ->writepage will magicly write not only that page but siblings too.

    buffer_head alredy has pointer to page so it is easy to get page from buffer

    page -> private -> next -> page -> private ...

    andf so on (sorry, but I did a study (for myself, trying to improve
    packet-writing) on that a half year ago, so I don't remember now much about

    What about that ?

    Best regards,
    Maxim Levitsky
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