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

[Avi Kivity <avi@redhat.com>]

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.

Fabrice's point also holds: if the guest is issuing many write
transactions for some reason, you don't want them hammering the disk and
killing your desktop performance if you're just developing, say, a new
filesystem.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.