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

[Dave Chinner <david@fromorbit.com>]

On Wed, Jan 04, 2012 at 02:53:37PM +0800, Shaohua Li wrote:
> 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!

Benchmark results?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com