The stripe cache allows us to avoid extra read/modify/write cycles
by caching the pages we read off the disk. Pages are cached when:
* They are read in during a read/modify/write cycle
* They are written during a read/modify/write cycle
* They are involved in a parity rebuild
Pages are not cached if we're doing a full stripe write. We're
assuming that a full stripe write won't be followed by another
partial stripe write any time soon.
This provides a substantial boost in performance for workloads that
synchronously modify adjacent offsets in the file, and for the parity
rebuild use case in general.
The size of the stripe cache isn't tunable (yet) and is set at 1024
entries.
Example on flash: dd if=/dev/zero of=/mnt/xxx bs=4K oflag=direct
Without the stripe cache -- 2.1MB/s
With the stripe cache 21MB/s
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
This builds on David Woodhouse's original Btrfs raid5/6 implementation.
The code has changed quite a bit, blame Chris Mason for any bugs.
Read/modify/write is done after the higher levels of the filesystem have
prepared a given bio. This means the higher layers are not responsible
for building full stripes, and they don't need to query for the topology
of the extents that may get allocated during delayed allocation runs.
It also means different files can easily share the same stripe.
But, it does expose us to incorrect parity if we crash or lose power
while doing a read/modify/write cycle. This will be addressed in a
later commit.
Scrub is unable to repair crc errors on raid5/6 chunks.
Discard does not work on raid5/6 (yet)
The stripe size is fixed at 64KiB per disk. This will be tunable
in a later commit.
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
Chris Mason