[RFC 0/3]block: An IOPS based ioscheduler

[Shaohua Li]

An IOPS based I/O scheduler

Flash based storage has some different characteristics against rotate disk.
1. no I/O seek.
2. read and write I/O cost usually is much different.
3. Time which a request takes depends on request size.
4. High throughput and IOPS, low latency.

CFQ iosched does well for rotate disk, for example fair dispatching, idle
for sequential read. It also has optimization for flash based storage (for
item 1 above), but overall it's not designed for flash based storage. It's
a slice based algorithm. Since flash based storage request cost is very
low, and drive has big queue_depth is quite popular now which makes
dispatching cost even lower, CFQ's slice accounting (jiffy based)
doesn't work well. CFQ doesn't consider above item 2 & 3.

FIOPS (Fair IOPS) ioscheduler is trying to fix the gaps. It's IOPS based, so
only targets for drive without I/O seek. It's quite similar like CFQ, but
the dispatch decision is made according to IOPS instead of slice.

The algorithm is simple. Drive has a service tree, and each task lives in
the tree. The key into the tree is called vios (virtual I/O). Every request
has vios, which is calculated according to its ioprio, request size and so
on. Task's vios is the sum of vios of all requests it dispatches. FIOPS
always selects task with minimum vios in the service tree and let the task
dispatch request. The dispatched request's vios is then added to the task's
vios and the task is repositioned in the sevice tree.

The series are orgnized as:
Patch 1: separate CFQ's io context management code. FIOPS will use it too.
Patch 2: The core FIOPS.
Patch 3: request read/write vios scale. This demontrates how the vios scale.

To make the code simple for easy view, some scale code isn't included here,
some not implementated yet.

TODO:
1. ioprio support (have patch already)
2. request size vios scale
3. cgroup support
4. tracing support
5. automatically select default iosched according to QUEUE_FLAG_NONROT.

Comments and suggestions are welcome!

Thanks,
Shaohua
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