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Re: [Qemu-devel] [PATCH 00/23] block migration: Fixes, cleanups and spee

From: Pierre Riteau
Subject: Re: [Qemu-devel] [PATCH 00/23] block migration: Fixes, cleanups and speedups
Date: Mon, 30 Nov 2009 20:34:48 +0100

On 30 nov. 2009, at 20:25, Jan Kiszka wrote:

> Pierre Riteau wrote:
>> On 30 nov. 2009, at 19:34, Anthony Liguori wrote:
>>> Jan Kiszka wrote:
>>>> This series is a larger rework of the block migration support qemu
>>>> recently gained. Besides lots of code refactorings the major changes
>>>> are:
>>>> - Faster restore due to larger block sizes (even if the target disk is
>>>>  unallocated)
>>>> - Off-by-one fixes in the block dirty tracking code
>>>> - Allow for multiple migrations (after cancellation or if migrating
>>>>  into a backup image)
>>>> - Proper error handling
>>>> - Progress reporting fixes: report to monitor instead of stdout, report
>>>>  sum of multiple disks
>>>> - Report disk migration progress via 'info migrate'
>>>> - Progress report during restore
>>>> One patch is directly taken from Pierre Riteau queue [1] who happend to
>>>> work on the some topic the last days, two more are derived from his
>>>> commits.
>>>> These patches make block migration usable for us. Still, there are two
>>>> more major improvements on my wish/todo list:
>>>> - Respect specified maximum migration downtime (will require tracking
>>>>  of the number of dirty blocks + some coordination with ram migration)
>>>> - Do not transfere unallocated disk space (also for raw images, ie. add
>>>>  bdrv_is_allocated support for the latter)
>>>> In an off-list chat, Liran additionally brought up the topic that RAM
>>>> migration should not start too early so that we avoid re-transmitting
>>>> dirty pages over and over again while the disk image is slowly beamed
>>>> over.
>>>> I hope we can join our efforts to resolve the open topics quickly, the
>>>> critical ones ideally before the merge window closes.
>>> That really needs to happen no later than the end of this week.
>>> So Pierre/Liran, what do you think about Jan's series?
>>> Regards,
>>> Anthony Liguori
>> I'm currently testing these patches. Here are a few issues I noticed, before 
>> I forget about them.
>> - "migrate -d -b tcp:dest:port" works, but "migrate -b -d tcp:dest:port" 
>> doesn't, although "help migrate" doesn't really specify ordering as 
>> important. But anyway I think Liran is working on a new version of the 
>> command.
> Saw that too. I think the monitor commands simply do very primitive
> option parsing so far. Should be addressed if the final format comes
> with this issue as well.
>> - We use bdrv_aio_readv() to read blocks from the disk. This function 
>> increments rd_bytes and rd_ops, which are reported by "info blockstats". I 
>> don't think this read operations should appear in VM activity, especially if 
>> this interface is used by libvirt to report VM stats (and draw graphs in 
>> virt-manager, etc.). Same for write stats.
> Ack.
>> - We may need to call bdrv_reset_dirty() _before_ sending the data, to be 
>> sure the block is not rewritten in the meantime (maybe it's an issue only 
>> with kvm?)
> Can you elaborate? Even in case of multi-threaded qemu, the iomutex
> should protect us here.

I only said that because I remember seeing this kind of behavior, but with ram 
migration on kvm.
As I'm not familiar with the I/O emulation in qemu, if you say that it's OK, no 

By multi-threaded, are you talking about the IO thread feature?

>> - I seem to remember that disk images with 0 size are now possible. I'm 
>> afraid we will hit a divide by zero in this case: "progress = 
>> completed_sector_sum * 100 / block_mig_state.total_sector_sum;"
> Although I don't see their use, it should be handled gracefully, likely
> by skipping such disks.

From a patch by Stefan Weil a few weeks ago:

> Images with disk size 0 may be used for
> VM snapshots, but not to save normal block data.
> It is possible to create such images using
> qemu-img, but opening them later fails.
> So even "qemu-img info image.qcow2" is not
> possible for an image created with
> "qemu-img create -f qcow2 image.qcow2 0".

I'm not sure if that concerns us...

Pierre Riteau -- http://perso.univ-rennes1.fr/pierre.riteau/

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