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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 19:50:03 +0100

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.
- 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.
- 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?)
- 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;"

Apart from that, it works quite fine. Still a few things to cleanup (e.g. 
unused constants) but much better than before.
However, I haven't tested the incremental transfer support at all yet. It's on 
my todo list.

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

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