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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Fri, 10 Oct 2008 07:26:00 -0500
User-agent: Thunderbird (X11/20080925)

Aurelien Jarno wrote:
On Thu, Oct 09, 2008 at 12:00:41PM -0500, Anthony Liguori wrote:


So to summarize, I think we should enable O_DSYNC by default to ensure that guest data integrity is not dependent on the host OS, and that practically speaking, cache=off is only useful for very specialized circumstances. Part of the patch I'll follow up with includes changes to the man page to document all of this for users.


While I agree O_DSYNC should be the defaults, I wonder if we should keep
the current behaviour available for those who want it. We can imagine
the following options:
  cache=off     O_DIRECT
  cache=read    O_DSYNC         (default)

Or maybe cache=off, cache=on, cache=wb. So that the default was cache=on which is write-through, or the user can choose write-back caching.

But that said, I'm concerned that this is far too confusing for users. I don't think anyone is relying on disk write performance when in write-back mode simply because the guest already has a page cache so writes are already being completed instantaneously from the application's perspective.


Anthony Liguori
  cache=on      0

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