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    Subject[PATCH 0/6][TAKE5] fallocate system call
    N O T E: 
    1) Only Patches 4/7 and 7/7 are NEW. Rest of them are _already_ part
    of ext4 patch queue git tree hosted by Ted.
    2) The above new patches (4/7 and 7/7) are based on the dicussion
    between Andreas Dilger and David Chinner on the mode argument,
    when later posted a man page on fallocate.
    3) All of these patches are based on 2.6.22-rc4 kernel and apply to
    2.6.22-rc5 too (with some successfull hunks, though - since the
    ext4 patch queue git tree has some other patches as well before
    fallocate patches in the patch series).

    Changes from Take4 to Take5:
    1) New Patch 4/7 implements new flags and values for mode
    argument of fallocate system call.
    2) New Patch 7/7 implements 2 (out of 4) modes in ext4.
    Implementation of rest of the (two) modes is yet to be done.
    3) Updated the interface description below to mention new modes
    being supported.
    4) Removed "extent overlap check" bugfix (patch 4/6 in TAKE4,
    since it is now part of mainline.
    5) Corrected format of couple of multi-line comments, which got
    missed in earlier take.

    Changes from Take2 to Take3:
    1) Return type is now described in the interface description
    2) Patches rebased to 2.6.22-rc1 kernel.

    ** Each post will have an individual changelog for a particular patch.

    fallocate() is a new system call being proposed here which will allow
    applications to preallocate space to any file(s) in a file system.
    Each file system implementation that wants to use this feature will need
    to support an inode operation called fallocate.

    Applications can use this feature to avoid fragmentation to certain
    level and thus get faster access speed. With preallocation, applications
    also get a guarantee of space for particular file(s) - even if later the
    the system becomes full.

    Currently, glibc provides an interface called posix_fallocate() which
    can be used for similar cause. Though this has the advantage of working
    on all file systems, but it is quite slow (since it writes zeroes to
    each block that has to be preallocated). Without a doubt, file systems
    can do this more efficiently within the kernel, by implementing
    the proposed fallocate() system call. It is expected that
    posix_fallocate() will be modified to call this new system call first
    and incase the kernel/filesystem does not implement it, it should fall
    back to the current implementation of writing zeroes to the new blocks.

    The system call's layout is:

    asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len);

    fd: The descriptor of the open file.

    mode*: This specifies the behavior of the system call. Currently the
    system call supports four modes - FA_ALLOCATE, FA_DEALLOCATE,
    FA_ALLOCATE: Applications can use this mode to preallocate blocks to
    a given file (specified by fd). This mode changes the file size if
    the preallocation is done beyond the EOF. It also updates the
    ctime in the inode of the corresponding file, marking a
    successfull allocation.
    FA_FA_RESV_SPACE: This mode is quite same as FA_ALLOCATE. The only
    difference being that the file size will not be changed.
    FA_DEALLOCATE: This mode can be used by applications to deallocate the
    previously preallocated blocks. This also may change the file size
    and the ctime/mtime. This is reverse of FA_ALLOCATE mode.
    FA_UNRESV_SPACE: This mode is quite same as FA_DEALLOCATE. The
    difference being that the file size is not changed and the data is
    also deleted.
    * New modes might get added in future.

    offset: This is the offset in bytes, from where the preallocation should

    len: This is the number of bytes requested for preallocation (from

    RETURN VALUE: The system call returns 0 on success and an error on
    failure. This is done to keep the semantics same as of

    sys_fallocate() on s390:
    There is a problem with s390 ABI to implement sys_fallocate() with the
    proposed order of arguments. Martin Schwidefsky has suggested a patch to
    solve this problem which makes use of a wrapper in the kernel. This will
    require special handling of this system call on s390 in glibc as well.
    But, this seems to be the best solution so far.

    Known Problem:
    mmapped writes into uninitialized extents is a known problem with the
    current ext4 patches. Like XFS, ext4 may need to implement
    ->page_mkwrite() to solve this. See:

    Since there is a talk of ->fault() replacing ->page_mkwrite() and also
    with a generic block_page_mkwrite() implementation already posted, we
    can implement this later some time. See:

    1> Implementation on other architectures (other than i386, x86_64,
    ia64, ppc64 and s390(x)).
    2> A generic file system operation to handle fallocate
    (generic_fallocate), for filesystems that do _not_ have the fallocate
    inode operation implemented.
    3> Changes to glibc,
    a) to support fallocate() system call
    b) to make posix_fallocate() and posix_fallocate64() call fallocate()

    Following patches follow:
    Patch 1/6 : fallocate() implementation on i386, x86_64 and powerpc
    Patch 2/7 : fallocate() on s390(x)
    Patch 3/7 : fallocate() on ia64
    Patch 4/7 : support new modes in fallocate
    Patch 5/7 : ext4: fallocate support in ext4
    Patch 6/7 : ext4: write support for preallocated blocks
    Patch 7/7 : ext4: support new modes
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      Last update: 2007-06-25 15:33    [W:0.027 / U:240.296 seconds]
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