On Wed, Dec 11, 2013 at 08:55:33PM +0000, Yaodong Yang wrote: > I have a quick question about the virtio inside qemu. When the user > application sends to a specific virtual disk a large number of read requests > in a very short time, where should these requests be queued? Inside the > virtqueue? virtqueue available ring or the underlying bdrv_read_aio()?
Requests take up descriptor space in the vring. Remember the vring consists of 3 things: 1. Descriptor space 2. Available ring 3. Used ring While the request is pending its vring descriptors will be in use, regardless of whether the head index is in the avail/used rings. > Currently, I run one guest vm with two virtual disks (one for ubuntu system, > the other is to serve the data access.) When the system is running, I can > check there are for instance 50 inflight requests to read or write the system > virtual disk. However, the inflight requests to the data virtual disk is > always one. Which is not reasonable, because inside the vm we calculated the > avery response time for each request is much larger than the time interval > between two adjacent requests. I don't understand this paragraph. But virtio-blk and virtio-scsi support multiple in-flight requests. The guest should be able to achieve queue depths of around 128 with virtio-blk. > My ultimate goal is to get the number of in-flight requests for a certain > virtual disk. Could someone give me some hints? Tools like iostat(1) can tell you about in-flight requests at the Linux block layer level. Stefan
