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

From: Mark Wagner
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Sun, 12 Oct 2008 20:27:28 -0400
User-agent: Thunderbird (X11/20080226)

Anthony Liguori wrote:
Mark Wagner wrote:

I do not believe that this means that the data is still sitting in the
host cache.  I realize it may not yet be on a disk, but, at a minimum,
I would expect that is has been sent to the storage controller.  Do you
consider the hosts cache to be part of the storage subsystem ?

Yes. And the storage subsystem is often complicated like this. Consider if you had a hardware iSCSI initiator. The host just sees a SCSI disk and when the writes are issued as completed, that simply means the writes have gone to the iSCSI server. The iSCSI server may have its own cache or some deep storage multi-level cached storage subsystem.

If you stopped and listened to yourself, you'd see that you are making my 

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


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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