From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cong Meng Subject: Re: [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive Date: Wed, 22 Aug 2012 19:04:49 +0800 Message-ID: <5034BCD1.9020603@linux.vnet.ibm.com> References: <1345537427-21601-1-git-send-email-mc@linux.vnet.ibm.com> <50334B51.6050900@redhat.com> <503357B2.5040901@linux.vnet.ibm.com> <50335F78.1030005@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <50335F78.1030005@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Paolo Bonzini Cc: stefanha@linux.vnet.ibm.com, zwanp@cn.ibm.com, linuxram@us.ibm.com, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On 08/21/2012 06:14 PM, Paolo Bonzini wrote: > Il 21/08/2012 11:52, Stefan Hajnoczi ha scritto: >>>>>> Using /sys/dev/block or /sys/dev/char seems easier, and lets you >>>>>> retrieve the parameters for block devices too. >>>>>> >>>> what do you mean with "block devices"? Using "/dev/sda" instead of >>>> "/dev/sg0"? > > Yes. > >>>>>> However, I'm worried of the consequences this has for migration. You >>>>>> could have the same physical disk accessed with two different HBAs, with >>>>>> different limits. So I don't know if this can really be solved at all. >>>>>> >>>> I know little about qemu migration now. The pending scsi commands will be >>>> saved and >>>> transfered to remote machine when starting migration? >> >> Passthrough is already a migration blocker if both hosts do not have >> access to the same LUNs. > > Yes, but requiring the exact same hardware may be too much. I'm trying > to understand the problem better before committing to a threefold > spec/qemu/kernel change. > > Cong, what is the limit that the host HBA enforces (and what is the > HBA)? What commands see a problem? Is it fixed by using scsi-block > instead of scsi-generic (if you can use scsi-block at all, i.e. it's not > a tape or similar device)? > I don't see real problem caused by the the queue limits actually. It's a bug which Stefan told me. > With scsi-generic, QEMU uses a bounce buffer for non-I/O commands to a > SCSI passthrough device, so the only problem in that case should be the > maximum segment size. This could change in the future, but max_segments > and max_sectors should not yet be a problem. about bounce buffer, do you meat the buffer allocated in scsi_send_command() of hw/scsi-generic.c? Cong. > > With scsi-block, QEMU will use read/write on the block device and the > host kernel will then obey the host HBA's block limits. QEMU will still > use a bounce buffer for non-I/O commands to a scsi-block device, but the > payload is usually small for non-I/O commands. > > Paolo > >> When both hosts do have access to the same LUNs it's possible to >> extract the block queue limits (using sysfs) and compare them. >> >> Today you can start QEMU with different image files on both hosts. >> Migration will appear to work but the disk image on the destination >> host could be junk. This is a similar case, I don't see a problem >> except that there should be a safety check (maybe at the libvirt >> level) to make this safe. >