Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

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On 02/12/2012 09:16 PM, James Bottomley wrote:
Well, no-one's yet answered the question I had about why.  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.

The biggest differences between virtio-blk and virtio-scsi are that:

1) how the feature set is defined. virtio-blk defines the feature set of the device through a shared spec between the guest and the host. The virtio-scsi spec does not define a feature set for the devices, only for the transport. Introducing new features in the guest does not need to be done specifically for virt, it can be done in generic code (sd.c). This results in a large feature set and at the same time a very stable spec.

Right now virtio-blk covers common usecases nicely. However, the Linux block layer _is_ growing support for new operations: discard is already there, write same is in the works, extended copy will also come in due time. Perhaps we'll add them to virtio-blk, perhaps not. If we will, we will have to modify the spec, the host implementation, and the guest drivers for each possible guest OS. virtio-scsi will support them transparently. Depending on your configuration, it might work without touching the host at all.


2) for disks with SCSI attachment, the native interface is exposed precisely as it is in the host. I think we had some misunderstanding WRT queue parameter discovery. My concern with virtio-blk's SG_IO support is more general than that. It is that SG_IO accesses the host disk, not the guest disk. They will have the same data, but they are effectively different disks. For example they might have different queue parameters, hence the misunderstanding.

People are mostly using the SG_IO interface for sane purposes. For example you can ping the storage with INQUIRY commands to detect problems on the NAS or SAN. For these usecases the difference does not matter. However, there _are_ worrisome usecases for SG_IO that people are looking at. For example installing vendor backup tools in their guests. These tools send vendor-specific commands to the disks. Nothing particularly insane about that, but we want them to do it using a saner interface than VIRTIO_BLK_T_SCSI_CMD.


On top of this, only virtio-scsi obviously will support devices such as tapes.

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.

It's not necessarily better. However error handling in the host may simply not be there. This is for example the case of NFS-based storage with the "hard" option.

Paolo
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