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Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk |
Date: |
Sat, 29 Jul 2006 17:04:44 +0100 |
User-agent: |
KMail/1.9.3 |
On Saturday 29 July 2006 15:59, Rik van Riel wrote:
> Fabrice Bellard wrote:
> > Hi,
> >
> > Using O_SYNC for disk image access is not acceptable: QEMU relies on the
> > host OS to ensure that the data is written correctly.
>
> This means that write ordering is not preserved, and on a power
> failure any data written by qemu (or Xen fully virt) guests may
> not be preserved.
I might be willing to accept this (or similar) patch if you made it
conditional on the guest having disabled write caching. I agree with Fabrice
that the performance impact it too severe to consider turning it on by
default.
The same problems occurs with many hardware RAID controllers, and even many
harddrives: fsync() only guarantees that the data has been passed to the
controller (in this case the host OS). If you need absolute reliability you
either need more flusing in your guest OS, disable the write cache, or
battery backup to make sure the IDE hardware (ie. host OS) doesn't die
unexpectedly.
Paul
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Fabrice Bellard, 2006/07/29
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Bill C. Riemers, 2006/07/29
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Jamie Lokier, 2006/07/30
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Jamie Lokier, 2006/07/30
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, andrzej zaborowski, 2006/07/31
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Jens Axboe, 2006/07/31
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, andrzej zaborowski, 2006/07/31
Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk, Jens Axboe, 2006/07/31