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    SubjectRe: [PATCH 4/5] tcm: Unify UNMAP and WRITE_SAME w/ UNMAP=1 subsystem plugin handling
    On Thu, 2010-10-14 at 02:03 +0200, Christoph Hellwig wrote:
    > On Wed, Oct 13, 2010 at 01:56:08PM -0700, Nicholas A. Bellinger wrote:
    > > > The parsing of the WRITE SAME and UNMAP CDBs is something the generic
    > > > CDB parsing code should do,
    > >
    > > Ok, so you are thinking about a seperate transport_emulate_write_same()
    > > and transport_emulate_unmap() called from
    > > transport_emulate_control_cdb(), right..?
    > More or less yes.

    Ok, then I shall convert transport_generic_[unmap,write_same]() which
    currently call blk_issue_discard() directly from IBLOCK code, and turn
    the ->do_discard() subsystem API op into the LBA+Range subsystem call
    with the underlying IBLOCK specific call to blk_issue_discard().

    > > > and just give a range of lists of lba/len
    > > > pairs to the ->discard method in the backed.
    > >
    > > Yes, these are already available from the passed struct
    > > se_task->task_lba and ->task_size values.
    > Not for UNMAP. WRITE SAME in it's various incarnations uses the
    > standard LBA/LEN encoding and you seem to parse it nicely. But for
    > UNMAP the lba/len pairs are in the command payload. To support things
    > genericly you'd need a standard way to pass them. If you want to
    > limit yourself to one lba/len pair for one the scheme could work,
    > though.

    Yes, this is what transport_generic_unmap() is currently doing when
    called from iblock_do_discard() an walks the received UNMAP payload.

    > > Yes, so the problem of trying to make this code generic (eg: outside of
    > > TCM subsystem plugins) is that blk_issue_discard() takes struct
    > > block_device, which means we the subsystem plugin has to locate struct
    > > block_device inside of non generic cide.
    > blk_issue_discard is in no way generic. It's 100% iblock code and
    > really doesn't belong into any other backend.


    > And btw,
    > blk_issue_discard is rather suboptimal even in iblock - it's a
    > synchronous function that will stall progress of the thread handling it.
    > If you want better performance you'll need to opencode the content of
    > it to allow an asynchronous completion handler. But given that discard
    > isn't really a critical feature at this point this could easily be
    > left for later with a comment.

    I have not gotten around to the async discard caller just yet, but this
    is straight-forward enough for the next round..

    > > So, then the main issue becomes FILEIO + block level discard and how to
    > > issue an blk_issue_discard() from struct fileio in the most sane way.
    > > If there is no sane way then I will just drop this bit, or just do the
    > > file level 'hole punch' that you are speaking about.
    > Right now there is no good way to do a block device discard or file
    > hole punch at the level where the file backend operates.

    Understood. In that case I will go ahead and drop the FILEIO discard
    support all together for .37 code, and we can revist as necessary down
    the road.



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