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Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes. |
Date: |
Tue, 19 Mar 2013 11:43:24 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130219 Thunderbird/17.0.3 |
Il 19/03/2013 11:06, George Dunlap ha scritto:
> On Mon, Mar 18, 2013 at 6:00 PM, Paolo Bonzini <address@hidden> wrote:
>> Il 18/03/2013 18:38, George Dunlap ha scritto:
>>>>>
>>>> This might be a difference between Xen and KVM. On Xen migration is
>>>> made to a server in a paused state, and it's only unpaused when
>>>> the migration to B is complete. There's a sort of extra handshake at
>>>> the end.
>>>
>>> I think what you mean is that all the memory is handled by Xen and the
>>> toolstack, not by qemu. The qemu state is sent as the very last thing,
>>> after all of the memory, and therefore (you are arguing) that qemu is
>>> not started, and the files cannot be opened, until after the migration
>>> is nearly complete, and certainly until after the file is closed on the
>>> sending side.
>>
>> That would be quite dangerous. Files aren't closed until after QEMU
>> exits; at this point whatever problem you have launching QEMU on the
>> destination would be unrecoverable.
>
> But if I understand your concern correctly, you were concerned about
> the following scenario:
> R1. Receiver qemu opens file
> R2. Something causes receiver kernel to cache parts of file (maybe
> optimistic read-ahead)
For some image formats, metadata is cached inside QEMU on startup.
There is a callback to invalidate QEMU's cache at the end of migration,
but that does not extend to the page cache.
> S1. Sender qemu writes to file
> S2. Sender qemu does final flush
> S3. Sender qemu closes file
> R3. Receiver reads stale blocks from cache
>
> Even supposing that Xen doesn't actually shut down qemu until it is
> started on the remote side, as long as the file isn't opened by qemu
> until after S2, we should be safe, right? It would look like this:
>
> S1. Sender qemu writes to file
> S2. Sender qemu does final flush
> R1. Receiver qemu opens file
> R2. Receiver kernel caches file
> S3. Sender qemu closes file
>
> This is all assuming that:
> 1. The barrier operations / write flush are effective at getting the
> data back on to the NFS server
> 2. The receiver qemu doesn't open the file until after the last flush
> by the sender.
>
> Number 1 has been tested by Alex I believe, and is mentioned in the
> changeset log; so if #2 is true, then we should be safe. I'll try to
> verify that today.
Thanks.
>> Even for successful migration, it would also be bad for downtime (QEMU
>> isn't exactly lightning-fast to start). And even if failure weren't
>> catastrophic, it would be a pity to transfer a few gigs of memory and
>> then find out that QEMU isn't present in the destination. :)
>
> Well, if qemu isn't present at the destination, that's definitely user
> error. :-) In any case, I know that he migrate can resume if it
> fails, so I suspect that the qemu is just paused on the sending side
> until the migration is known to complete. As long as the last write
> was flushed to the NFS server before the receiver opens the file, we
> should be safe.
Note that the close really must happen before the next open. Otherwise
the file metadata might not be up-to-date on the destination, too.
Paolo
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., (continued)
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/18
- Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/18
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.,
Paolo Bonzini <=
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/19
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Stefano Stabellini, 2013/03/20
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., George Dunlap, 2013/03/20
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Paolo Bonzini, 2013/03/20
- Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes., Alex Bligh, 2013/03/20