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    SubjectRe: Pe: [PATCH v5 1/3] virtio-scsi: first version
    On 02/13/2012 02:40 PM, Nicholas A. Bellinger wrote:
    > Hi Dor, James& Co,
    > On Mon, 2012-02-13 at 09:57 +0200, Dor Laor wrote:
    >> On 02/13/2012 09:05 AM, Christian Borntraeger wrote:
    >>> On 12/02/12 21:16, James Bottomley wrote:
    >>>> Well, no-one's yet answered the question I had about why.
    >>> Just to give one example from a different angle:
    >>> In the big datacenters tape libraries are still very important, and lots
    >>> of them have a scsi attachement. virtio-blk certainly is not the right
    >>> way to handle those. Furthermore it seems even pretty hard to craft
    >>> a virtio-tape since most of those libraries have vendor specific library
    >>> controls (via sg). We would need to duplicate scsi generic (hint, hint :-)
    >>>> virtio-scsi seems to be a basic duplication of virtio-blk except that it seems to
    >>>> fix some problems virtio-blk has. Namely queue parameter discover,
    >>>> which virtio-blk doesn't seem to do. There may also be a reason to cut
    >>>> the stack lower down. Error handling is most often cited for this, but
    >>>> no-one's satisfactorily explaned why it's better to do error handling in
    >>>> the guest instead of the host.
    >>>> Could someone please explain to me why you can't simply fix virtio-blk?
    >>> I dont think that virtio-scsi will replace virtio-blk everywhere. For non-scsi
    >>> block devices, image files or logical volumes virtio-blk seems to be the right
    >>> approach, I think.
    >> +1
    >> virtio-scsi is superior w.r.t:
    >> - Device support: tapes, cdroms, other
    > AFAICT any type of non TYPE_DISK struct scsi_device passthrough is going
    > to currently require virtio-scsi in order to work.
    >> - Does guest-host mapped multipath
    > The logic that comes with target_core_fabric_configfs.c and the native
    > target control plane gives a host-side (tcm_vhost) fabric driver generic
    > explict/implict ALUA multipath support by default.
    > I think there are some interesting possibilities for paravirtualized
    > ALUA multipath.. 8-)
    >> - Supports plenty of virtual disks mapped to the guest w/o need for a
    >> pci slot per each virtio-blk
    > Ouch, virtio-blk lacks multi-lun per pci slot support..?

    Only if you use the pci multi-function option but that kills standard
    hot unplug

    >> - offload fancy/new/sophisticated scsi commands from the guest to the
    >> storage array w/o need for qemu implementation. Example XCOPY.
    > ...
    >> There are some more goodies like ability to support windows guest
    >> clustering w/o hacky versions of scsi pass through over virtio-blk.
    >> virtio-blk is also a candidate to change the request based towards bio
    >> based implementation, so sticking to it does not buy us too much.
    > MSFT cluster guests that require SPC-3 PR support can run today with
    > tcm_loop LLD SCSI LUNs + SG_IO/BSG + right megasas QEMU HBA emulation,
    > but I do agree this would be better served by virtio-scsi for guests
    > that require SPC-3 PR support or passthrough.
    > --nab
    > --
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      Last update: 2012-02-13 13:57    [W:0.032 / U:12.556 seconds]
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