On 06/16/2011 08:03 PM, Sasha Levin wrote:
On Thu, 2011-06-16 at 17:50 -0500, Anthony Liguori wrote: Each virtio-blk device can process requests regardless of other virtio-blk devices, which means that we can do parallel requests for devices. Within each device, we support parallel requests in the sense that we do vectored IO for each head (which may contain multiple blocks) in the vring, we don't do multiple heads because when I've tried adding AIO
A scatter/gather list isn't multiple requests, it's just one. So you handle one request at a time ATM. There's nothing with that, but there's no use in saying "we support it in the sense..." :-)
I've noticed that at most there are 2-3 possible heads - and since it points to the same device it doesn't really help running them in parallel.
Sure it does. If you use the host page cache (and you do), then if you have two requests, A and B, and request A requires a disk access and request B can be satisfied from the page cache, then being able to submit both requests means that you can return B almost immediately instead of stalling out to finish A before starting B.
Not to mention that modern disks work better with multiple in flight requests because they have their own cache and reordering algorithms in the drives cache. With RAID and higher end storage devices, a single device may map to multiple spindles. The only way to have them all spin at once is to submit parallel requests.
Regards, Anthony Liguori -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
