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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: Sun, 12 Oct 2008 20:21:24 -0500
User-agent: Thunderbird (X11/20080925)

Mark Wagner wrote:
If you stopped and listened to yourself, you'd see that you are making my point...

AFAIK, QEMU is neither designed nor intended to be an Enterprise Storage Array, I thought this group is designing a virtualization layer. However, the persistent argument is that since Enterprise Storage products will often acknowledge a write
before the data is actually on the disk, its OK for QEMU to do the same.

I think you're a little lost in this thread. We're going to have QEMU only acknowledge writes when they complete. I've already sent out a patch. Just waiting a couple days to let everyone give their input.

had a similar design to Enterprise Storage with redundancy, battery backup, etc, I'd be fine with it, but you don't. QEMU is a layer that I've also thought was suppose to be small, lightweight and unobtrusive that is silently putting everyones data
at risk.

The low-end iSCSI server from EqualLogic claims:
    "it combines intelligence and automation with fault tolerance"
"Dual, redundant controllers with a total of 4 GB battery-backed memory"

AFAIK QEMU provides neither of these characteristics.

So if this is your only concern, we're in violent agreement. You were previously arguing that we should use O_DIRECT in the host if we're not "lying" about write completions anymore. That's what I'm opposing because the details of whether we use O_DIRECT or not have absolutely nothing to do with data integrity as long as we're using O_DSYNC.


Anthony Liguori


The fact that the virtualization layer has a cache is really not that unusual.
Do other virtualization layers lie to the guest and indicate that the data has successfully been ACK'd by the storage subsystem when the data is actually
still in the host cache?



Anthony Liguori

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