Re: [Qemu-devel] [PATCH 0/9] Add full scatter-gather support for SCSI generic devices

[Paolo Bonzini]

On 17/12/2015 14:10, Alex Pyrgiotis wrote:
>>>>> Which commands have large payloads and are on the data path, for
>>>>> scsi-block?  Or is the use case just scsi-generic (e.g. tape devices?)?
>
> If I understand correctly, what you're saying is that if "scsi-block" is
> started with "cache=writeback" and internally uses ioctl()s to bypass
> the page cache, why not set "cache=none" beforehand and use
> readv()/writev()?
> 
> This is a valid suggestion, but this patch does not target only the
> "scsi-block" device type. Its purpose is to allow faster read/writes via
> ioctl()s, either to a "scsi-block" device or to a "scsi-generic" device.
> Note that the latter device type can only use ioctl()s, so it cannot
> benefit from the readv()/writev() DMA interface and currently has to use
> a bounce buffer.

Okay, so that answers my questions; there is still a valid use case for
e.g. tape devices, and of course for when someone forgets to use scsi-block.

Paolo

>> We can improve the code to print a warning if you don't.  (It needs some
>> care: iscsi never caches, independent of the cache= argument, so we
>> don't want to warn for it.  But it can be done).
> 
> I wasn't particularly concerned about that issue. I'd may prefer if this
> was explicitly addressed in the QEMU doc, under the "cache=" section,
> but that's a different discussion.
> 
> Thanks,
> Alex
>