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Subject[00/17] Large Blocksize Support V3
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

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