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Subject[00/37] Large Blocksize Support V4
V3->V4
- It is possible to transparently make filesystems support larger
blocksizes by simply allowing larger blocksizes in set_blocksize.
Remove all special modifications for mmap etc from the filesystems.
This now makes 3 disk based filesystems that can use larger blocks
(reiser, ext2, xfs). Are there any other useful ones to make work?
- Patch against 2.6.22-rc4-mm2 which allows the use of Mel's antifrag
logic to avoid fragmentation.
- More page cache cleanup by applying the functions to filesystems.
- Disable bouncing when the gfp mask is setup.
- Disable mmap directly in mm/filemap.c to avoid filesystem changes
while we have no mmap support for higher order pages.

RFC 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.

RFC 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.
Using 64k block size will reduce the number of blocks to be managed
by a factor of 16 and produce much denser and contiguous metadata.

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 becomes possible. Note that this also means that
some filesystems are already capable of working with blocksizes of
up to 64k (ext2, XFS) which is currently only available on a select
few arches. This patchset enables that functionality on all arches.
There are no special modifications needed to the filesystems. The
set_blocksize() function call will simply support a larger blocksize.

7. VM scalability
Large block sizes mean less state keeping for the information being
transferred. For a 1TB file one needs to handle 256 million page
structs in the VM if one uses 4k page size. A 64k page size reduces
that amount to 16 million. If the limitation in existing filesystems
are removed then even higher reductions become possible. For very
large files like that a page size of 2 MB may be beneficial which
will reduce the number of page struct to handle to 512k. The variable
nature of the block size means that the size can be tuned at file
system creation time for the anticipated needs on a volume.

8. IO scalability
The IO layer will receive large blocks of contiguious memory with
this patchset. This means that less scatter gather elements are needed
and the memory used is guaranteed to be contiguous. Instead of having
to handle 4k chunks we can f.e. handle 64k chunks in one go.

Dave Chinner measures a performance increase of 50% when going to 64k
blocksize with XFS.

How to make this work:

1. Apply this patchset on top of 2.6.22-rc4-mm2
2. Enable LARGE_BLOCKSIZE Support
3. compile kernel

In order to use a filesystem with a higher order it needs to be formatted
with larger blocksize. This is done using the mkfs.xxx tool for each
filesystem. The existing tools work without modification. They may just
warn you that the blocksize you specify is not supported on your particular
architecture. Ignore that warning since this is no longer true after you have
applied this patchset.


Tested file systems:

Filesystem Max Blocksize Changes

Reiserfs 8k Page size functions
Ext2 64k Page size functions
XFS 64k Page size functions / Remove PAGE_SIZE check
Ramfs MAX_ORDER Parameter to specify order

Todo/Issues:

- There are certainly numerous issues with this patch. I have only tested
copying files back and forth, volume creation etc. Others have run
fsxlinux on the volumes. The missing mmap support limits what can be
done for now.

- Antifragmentation in mm does address some fragmentation issues (typically
works up to 32k blocksize). However, large orders lead to fragmentation of
the movable sections. Seems that we need Mel's memory compaction to support
even larger orders. How memory compaction impacts performance still has to
be determined.

- Support for bouncing pages.

- Remove PAGE_CACHE_xxx constants after using page_cache_xxx functions
everywhere. But that will have to wait until merging becomes possible.
For now certain subsystems (shmem f.e.) are not using these functions.
They will only use order 0 pages.

- Support for non harddisk based filesystems. Remove the pktdvd etc
layers needed because the VM current does not support sufficiently
large blocksizes for these devices. Look for other places in the kernel
where we have similar issues.

- Mmap read support

Its likely easier to do restricted read only mmap support first. That
would enable running executables off the filesystems with large block
size.

- Full mmmap support

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