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Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive |
Date: |
Tue, 21 Aug 2012 12:14:16 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120717 Thunderbird/14.0 |
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.
- [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Cong Meng, 2012/08/21
- [Qemu-devel] [PATCH 2/2 v1] virtio-scsi: set per-LUN queue limits for sg devices, Cong Meng, 2012/08/21
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Paolo Bonzini, 2012/08/21
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Cong Meng, 2012/08/21
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Stefan Hajnoczi, 2012/08/21
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Cong Meng, 2012/08/22
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Paolo Bonzini, 2012/08/22
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Stefan Hajnoczi, 2012/08/22
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Paolo Bonzini, 2012/08/22
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Cong Meng, 2012/08/23
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Paolo Bonzini, 2012/08/23
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Stefan Hajnoczi, 2012/08/23
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Paolo Bonzini, 2012/08/23
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Stefan Hajnoczi, 2012/08/23
- Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive, Nicholas A. Bellinger, 2012/08/23