RAID5: batch adjacent full stripe write
stripe cache is 4k size. Even adjacent full stripe writes are handled in 4k unit. Idealy we should use big size for adjacent full stripe writes. Bigger stripe cache size means less stripes runing in the state machine so can reduce cpu overhead. And also bigger size can cause bigger IO size dispatched to under layer disks. With below patch, we will automatically batch adjacent full stripe write together. Such stripes will be added to the batch list. Only the first stripe of the list will be put to handle_list and so run handle_stripe(). Some steps of handle_stripe() are extended to cover all stripes of the list, including ops_run_io, ops_run_biodrain and so on. With this patch, we have less stripes running in handle_stripe() and we send IO of whole stripe list together to increase IO size. Stripes added to a batch list have some limitations. A batch list can only include full stripe write and can't cross chunk boundary to make sure stripes have the same parity disks. Stripes in a batch list must be in the same state (no written, toread and so on). If a stripe is in a batch list, all new read/write to add_stripe_bio will be blocked to overlap conflict till the batch list is handled. The limitations will make sure stripes in a batch list be in exactly the same state in the life circly. I did test running 160k randwrite in a RAID5 array with 32k chunk size and 6 PCIe SSD. This patch improves around 30% performance and IO size to under layer disk is exactly 32k. I also run a 4k randwrite test in the same array to make sure the performance isn't changed with the patch. Signed-off-by:Shaohua Li <shli@fusionio.com> Signed-off-by:
NeilBrown <neilb@suse.de>
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