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

From: Jens Axboe
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Fri, 17 Oct 2008 15:14:42 +0200

On Fri, Oct 10 2008, Avi Kivity wrote:
> Anthony Liguori wrote:
> >>
> >> This isn't entirely true. With IDE devices you don't have command
> >> queueing, so it's easy for a large write to stall subsequent reads
> >> for a relatively long time.
> >> I'm not sure how much this effects qemu, but I've definitely seen it
> >> happening on real hardware.
> >>   
> >
> > I think that suggests we should have a cache=wb option and if people
> > report slow downs with IDE, we can observe if cache=wb helps.  My
> > suspicion is that it's not going to have a practical impact because as
> > long as the operations are asynchronous (via DMA), then you're getting
> > native-like performance.
> >
> > My bigger concern is synchronous IO operations because then a guest
> > VCPU is getting far less time to run and that may have a cascading
> > effect on performance.
> IDE is limited to 256 sectors per transaction, or 128KB.  If a sync
> transaction takes 5 ms, then your write rate is limited to 25 MB/sec. 
> It's much worse if you're allocating qcow2 data, so each transaction is
> several sync writes.

No it isn't, even most IDE drives support lba48 which raises that limit
to 64K sectors, or 32MB.

Jens Axboe

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