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    SubjectRe: [PATCH 0/7] discard support revisited
    On Sun, Aug 30, 2009 at 03:17:19PM -0500, James Bottomley wrote:
    > > Good question. Latest I had heard was that at least one array vendor
    > > prefers the WRITE SAME. To me it looks like the much saner interface
    > > for the OS, so unless there are arrays that strongly prefer UNMAP or
    > > we need to make use of the multiple extends feature in it I'd go with
    > > WRITE SAME as first choice.
    >
    > So, since their respective names are on the proposals, it's no real
    > secret that EMC are pushing WRITE_SAME and Netapp UNMAP, but they are
    > both working together on this. I've already communicated to T10 via
    > intermediaries that we'd like only a single implementation for this,
    > please. However, failing that, the current situation where we know from
    > an inquiry that the array supports thin provisioning, but don't know
    > whether it supports WRITE_SAME or UNMAP until we get a command failure
    > is unacceptable.
    >
    > If we could get some good solid implementation evidence that WRITE_SAME
    > is much easier for an OS than UNMAP, that might help with the T10
    > deliberations.

    As I've recently worked on all sides of the discard battle (filesystem
    support, initiator support, and target support) here are my notes:


    - WRITE_SAME is extremly nice to implement for both the initiator and
    target. It has the LBA and len exactly in the same place as normal
    16 byte commands, the payload length is fixed to one block, which
    we can allocate once and zero so that we don't even need any memory
    allocations for this command in the initiator.
    - UNMAP is a pain to implement in both initiator and target. Not
    actuall having the LBA/len information in the cdb but in the payload
    is at least a minor incovenience in the initator, and quite annoying
    in the target as we now need to process payload data in the fastpath,
    which we otherwise only do for slow path CDBs. This will be
    especially bad for split kernel/user target implementations.

    Now the weird design of UNMAP of course has a rather (besides some
    apparent pissing contest at NetApp about who can't come with the worst
    possible protocol specifications, whose results can be seen in NFSv4
    and iSer), and that is that it allows dicarding of multiple
    discontinguous ranges. Doing so is really bad for the filesystem as
    it requires it to track multiple outstanding discard requests, which
    requires locking, and book keeping to make sure we do not re-use these
    blocks before they are discarded.

    And at least for my target design it does not provide any measureable
    benefits at all, the discard operations are mapped to a hole punch
    ioctl on a filesystem, which has a constant basic overhead for each
    region punched (synchronous transaction commit) and a small linear
    cost per extent removed. The only benefit of the multiple rangs unmap
    would be a saving of protocol roundtrips.

    Now that is interestingly actually a downside at least for my still
    rather dumb target implementation with a typical Linux filesystem
    workload on the initiator side. If we actually do a lot different unmap
    operations in a single unmap command it can start to take significant
    amounts of time, and do to Linux waiting for queue drains frequently
    due to the barrier implementations we will end up waiting for the unmap
    command.



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