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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapsh

From: Denis V. Lunev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI
Date: Tue, 13 Feb 2018 18:29:12 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 02/13/2018 06:27 PM, Dr. David Alan Gilbert wrote:
> * Roman Kagan (address@hidden) wrote:
>> On Tue, Feb 13, 2018 at 03:05:03PM +0000, Dr. David Alan Gilbert wrote:
>>> * Denis V. Lunev (address@hidden) wrote:
>>>> On 02/13/2018 05:59 PM, Dr. David Alan Gilbert wrote:
>>>>> * Daniel P. Berrangé (address@hidden) wrote:
>>>>>> That doesn't seem practical unless you can instantaneously write out
>>>>>> the entire guest RAM to disk without blocking, or can somehow snapshot
>>>>>> the RAM so you can write out a consistent view of the original RAM,
>>>>>> while the guest continues to dirty RAM pages.
>>>>> People have suggested doing something like that with userfault write
>>>>> mode; but the same would also be doable just by write protecting the
>>>>> whole of RAM and then following the faults.
>>>> nope, userfault fd does not help :( We have tried, the functionality is not
>>>> enough. Better to have small extension to KVM to protect all memory
>>>> and notify QEMU with accessed address.
>>> Can you explain why? I thought the write-protect mode of userfaultfd was
>>> supposed to be able to do that; cc'ing in Andrea
>> IIRC it would if it worked; it just didn't when we tried.
> Hmm that doesn't seem to be the ideal reason to create new KVM
> functionality, especially since there were a bunch of people wanting the
> userfaultfd-write mode for other uses.
This does not seems to work in practice. We status with that is the
same for more than a year and a half AFAIR. Thus there is a lot
of sense to create something feasible as async snapshots are
badly wanted.


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