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

[Mark Wagner <mwagner@redhat.com>]

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 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. 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 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.

-mark

> 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?


-mark
> 
> Regards,
> 
> Anthony Liguori
> 
>