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Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command |
Date: |
Tue, 21 May 2013 10:30:22 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4 |
Il 21/05/2013 09:31, Stefan Hajnoczi ha scritto:
> On Mon, May 20, 2013 at 09:23:43AM +0200, Paolo Bonzini wrote:
>> Il 20/05/2013 08:24, Stefan Hajnoczi ha scritto:
>>>>> You only need to fdatasync() before every guest flush, no?
>>> No, you need to set the dirty bit before issuing the write on the
>>> host. Otherwise the image data may be modified without setting the
>>> appropriate dirty bit. That would allow data modifications to go
>>> undetected!
>>
>> But data modifications can go undetected until the guest flush returns,
>> can't they?
>
> You are thinking about it from the guest perspective - if a flush has
> not completed yet then there is no guarantee that the write has reached
> disk.
>
> But from a host perspective the dirty bitmap should be conservative so
> that the backup application can always restore a bit-for-bit identical
> copy of the disk image. It would be weird if writes can sneak in
> unnoticed.
True, but that would happen only in case the host crashes. Even for a
QEMU crash the changes would be safe, I think. They would be written
back when the persistent dirty bitmap's mmap() area is unmapped, during
process exit.
Paolo
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, (continued)
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/17
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/20
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/20
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/23
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/23
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/24
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/24