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Re: [Qemu-devel] [PATCH] Flush pending AIO on reboot and shutdown.


From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] Flush pending AIO on reboot and shutdown.
Date: Wed, 13 Aug 2008 14:04:55 -0500
User-agent: Thunderbird 2.0.0.16 (X11/20080723)

Gleb Natapov wrote:
On Wed, Aug 13, 2008 at 09:40:34PM +0300, Avi Kivity wrote:
Gleb Natapov wrote:
but what bother me is that we
consciously drop user data that we can easily save. Why? Real HW tries
hard to save every bit of user data and we just decided to drop it. The
difference between cancel or complete a request may be corrupted or not
corrupted file system after a crash.

If a guest didn't wait for an I/O to complete, it shouldn't expect it to be on disk.
If you'll save your swiss bank account details in file and suddenly
your guest will crash, you'll surely appreciate if qemu will save it
for you :)

Of course, we can have the IDE layer wait instead of cancelling, which will get the request onto the disk.

That is the current approach, but the wait is done for all block IO not
just IDE.

If the guest hasn't seen confirmation that the data is on disk, as far as the guest is concerned, it's not on disk.

If you have a journalled file system, for instance, unless it receives that notification, it's going to replay whatever has happened in the journal since the last successful write operation. Practically speaking, I don't think you're saving data.

Regards,

Anthony Liguori

--
                        Gleb.







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