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)? 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. 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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
