qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk


From: Rik van Riel
Subject: Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk
Date: Sat, 29 Jul 2006 12:22:13 -0400
User-agent: Thunderbird 1.5.0.4 (X11/20060614)

Paul Brook wrote:
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.

Easy to do with the fsync infrastructure, but probably not worth
doing since people are working on the AIO I/O backend, which would
allow multiple outstanding writes from a guest.  That, in turn,
means I/O completion in the guest can be done when the data really
hits disk, but without a performance impact.

--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan




reply via email to

[Prev in Thread] Current Thread [Next in Thread]