On Sep 03, 2007  20:03 +0200, Jan Kara wrote:

  I've finally got to writing up some proposal how could look system calls
allowing for online filesystem defragmentation and generally moving file
blocks around for improving performance. Comments are welcome.

int sys_movedata(int datafd, int spacefd, loff_t from, size_t len)
   The call takes blocks used to carry data starting at offset @from of length
@len in @spacefd and places them instead of corresponding blocks in @datafd.

Calling these "@spacefd" and "@datafd" is a bit confusing.  How about "@srcfd"
and "@tgtfd" instead?  For defragmentation, are you planning to have @datafd
be the "real" inode and "@spacefd" be the temporary inode with defragged data,
or the reverse?  It isn't really clear.

Data is copied from @datafd to newly spliced data blocks. If @spacefd contains
a hole in the specified interval, a hole is created also in @datafd in the
corresponding place. A data block from @spacefd and also replace a hole in
@datafd - zeros are copied to such data block. @from and @len should be
multiples of filesystem block size (otherwise EINVAL is returned). Data blocks
from @datafd in the interval are released, a hole is created in @spacefd.

This is mostly clear except the last sentence.  I would think that the data
blocks in @datafd are kept, getting a copy of the data, while those in
@spacefd are released?

  Another possibility would be to just replace data blocks without any copying
of data (that would have to be done by the caller to before calling
sys_movedata()). The problem here is how to avoid data loss if someone writes
to the file after userspace has copied the data and before sys_movedata() is
called.

Isn't that true in any case?

ssize_t sys_allocate(int fd, int mode, loff_t goal, ssize_t len)
  Allocate new space to file @fd at offset defined by file position.  Both file
offset and @len should be a multiple of filesystem block size. The whole
interval must not contain any allocated blocks. If the interval extends past
EOF, the file size is changed accordingly.  @mode defines a way the filesystem
will search for blocks. @mode is a bitwise OR of the following flags:
  ALLOC_FIXED_START - allocation must start at @goal; if not specified, @goal
is just a hint where to start an allocation
  ALLOC_FIXED_LEN - allocate exactly space for @len; if not specified, upto
@len bytes may be allocated.
  ALLOC_CONTINGUOUS - allocation must be one continguous run of blocks

How is this much different than sys_fallocate()?

int sys_get_free_blocks(const char *fs, loff_t start, loff_t end, int count,
  struct alloc_extent *space)

One alternate possibility is to call the proposed FIEMAP on the block device,
to return lists of free/used extents?  We have a version of that patch for
ext4 and integration into filefrag, so it would be nice to avoid making up
yet another API/tool if that one is sufficient.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.