OK, Thank you very much for your detailed explanation. But I have another question about the big change from qemu-1.5.3 to qemu-1.6.0-rc0.
When I use ramdisk for IO performance testing, the result is as follows. [fio-test] rw bs iodepth jobs bw iops ------------------------------------------------------------------------------------ qemu-1.5.3 read 4k 32 1 285MB/s 73208 qemu-1.6.0-rc0 read 4k 32 1 253MB/s 64967 And virtio-blk is the same. I know there are so many differences between qemu-1.5 and qemu-1.6, but I am confused about what new features impact the performance so much. Do you know it? On 2015-2-3 16:49, Paolo Bonzini wrote: > On 03/02/2015 03:56, Wangting (Kathy) wrote: >> Sorry, I find that the patch of "virtio-scsi: Optimize virtio_scsi_init_req" >> can slove this problem. > > Great that you could confirm that. :) > >> By the way, can you tell me the reason of the change about cdb and sense? > > cdb and sense are variable-size items. ANY_LAYOUT support changed > VirtIOSCSIReq: instead of having a pointer to the request, it copies the > request from guest memory into VirtIOSCSIReq. This is required because > the request might not be contiguous in guest memory. And because the > request and response headers (e.g. VirtIOSCSICmdReq and > VirtIOSCSICmdResp) are included by value in VirtIOSCSIReq, the > variable-sized fields have to be treated specially. > > Only one of them can remain in VirtIOSCSIReq, because you cannot have a > flexible array member (e.g. "uint_8 sense[];") in the middle of a struct. > > cdb is always used, so it is chosen for the variable-sized part of > VirtIOSCSIReq: cdb was simply moved from VirtIOSCSICmdReq to VirtIOSCSIReq. > > Instead, requests that complete with sense data are not a fast path. > Hence sense is retrieved from the SCSIRequest, and > virtio_scsi_command_complete copies it into the guest buffer via > scsi_req_get_sense + qemu_iovec_from_buf. > > Paolo > >
