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    SubjectRe: [RFC 0/3]block: An IOPS based ioscheduler
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    On Wed, 2012-01-04 at 18:19 +1100, Dave Chinner wrote:
    > 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?
    I didn't have data yet. The patches are still in earlier stage, I want
    to focus on the basic idea first.

    Thanks,
    Shaohua



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