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Re: [Qemu-devel] Qemu and Changed Block Tracking

From: Peter Lieven
Subject: Re: [Qemu-devel] Qemu and Changed Block Tracking
Date: Fri, 24 Feb 2017 08:59:05 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Am 23.02.2017 um 20:34 schrieb John Snow:
> On 02/23/2017 09:29 AM, Peter Lieven wrote:
>> Am 22.02.2017 um 22:17 schrieb John Snow:
>>> On 02/22/2017 03:45 AM, Peter Lieven wrote:
>>>> Am 21.02.2017 um 22:13 schrieb John Snow:
>>>>> On 02/21/2017 07:43 AM, Peter Lieven wrote:
>>>>>> Hi,
>>>>>> is there anyone ever thought about implementing something like VMware
>>>>>> CBT in Qemu?
>>>>>> https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1020128
>>>>>> Thanks,
>>>>>> Peter
>>>>> A bit outdated now, but:
>>>>> http://wiki.qemu-project.org/Features/IncrementalBackup
>>>>> and also a summary I wrote not too far back (PDF):
>>>>> https://drive.google.com/file/d/0B3CFr1TuHydWalVJaEdPaE5PbFE
>>>>> and I'm sure the Virtuozzo developers could chime in on this subject,
>>>>> but basically we do have something similar in the works, as eblake
>>>>> says.
>>>> Hi John, Hi Erik,
>>>> thanks for your feedback. Are you both the ones working primary on
>>>> this topic?
>>>> If there is anything to review or help needed, please let me know.
>>> I've been working on incremental backups; Fam and I now co-maintain
>>> block/dirty-bitmap.c.
>>> Vladimir Sementsov-Ogievskiy has been working on bitmap persistence and
>>> migration from Virtuozzo; as well as the NBD specification amendment to
>>> allow us to fleece images with dirty bitmaps.
>>> (Check the wiki and the whitepaper I linked!)
>>> Eric has been guiding the review process for the NBD side of things.
>>>> My 2 cents:
>>>> I thing I had in mind if there is no image fleecing available, but
>>>> fetching the dirty bitmap
>>>> from external would be a feauture to put a write lock on a block device.
>>>> Write lock means, drain all pending writes and queue all further
>>>> writes until unlock (as if they
>>>> were throttled to zero). This could help fetch consistent backups
>>>> from storage device (thinking of iSCSI SAN) without
>>>> the help of the hypervisor to actually transfer data (no load in the
>>>> frontend network or the host). What would further
>>>> be needed is a write generation for each block, not just only a dirty
>>>> bitmap.
>>>> In this case something like this via QMP (and external software)
>>>> should work:
>>>> ---8<---
>>>>   gen =  write generation of last backup (or 0 for full backup)
>>>>   do {
>>>>       nextgen = fetch current write generation (via QMP)
>>> As Eric said, there's a lot of hostility to using QMP as a metadata
>>> transmission protocol.
>>>>       dirtymap = send all block whose write generation is greater
>>>> than 'gen' (via QMP)
>>>>       dirtycnt = 0
>>>>       foreach block in dirtymap {
>>>>                 copy to backup via external software
>>>>                 dirtycnt++
>>>>       }
>>>>       gen = nextgen
>>>>   } while (dirtycnt < X)         <--- to achieve this a thorttling or
>>>> similar might be needed
>>>> fsfreeze (optional)
>>>> write lock (via QMP)
>>>> backupgen = fetch current write generation (via QMP)
>>>> dirtymap = send all block whose write generation is greater than
>>>> 'gen' (via QMP)
>>>> foreach block in dirtymap {
>>>>                 copy to backup via external software
>>>> }
>>>> unlock (via QMP)
>>>> fsthaw (optional)
>>>> --->8---
>>>> As far as I understand CBT in VMware is not just only a dirty bitmap,
>>>> but also a write generation tracking for blocks (size 64kb or whatever)
>>> I think at the moment I'm worried about getting the basic features out
>>> the door, but I'm not opposed to adding fancier features if there's
>>> justification or demand for them.
>> Sure, the basic features are most important. I was just thinking of the
>> above scenario to interact with a NAS and have Qemu's "help"
>> to create incremental backups.
>> Peter
> If you get the chance to read the white paper I linked to you, please
> let me know which use cases we might not be able to cover that you feel
> other programs might offer.

Will do. Only have had a short glimpse yet.

> I can also make a point to CC you on future upstream discussions as they
> happen.
Yes, please.


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