Re: Performance of SCST versus STGT

[Vladislav Bolkhovitin <vst@vlnb.net>]

FUJITA Tomonori wrote:
> On Tue, 22 Jan 2008 14:33:13 +0300
> Vladislav Bolkhovitin <vst@vlnb.net> wrote:
> 
> 
>>FUJITA Tomonori wrote:
>>
>>>The big problem of stgt iSER is disk I/Os (move data between disk and
>>>page cache). We need a proper asynchronous I/O mechanism, however,
>>>Linux doesn't provide such and we use a workaround, which incurs large
>>>latency. I guess, we cannot solve this until syslets is merged into
>>>mainline.
>>
>>Hmm, SCST also doesn't have ability to use asynchronous I/O, but that 
>>doesn't prevent it from showing good performance.
> 
> 
> I don't know how SCST performs I/Os, but surely, in kernel space, you
> can performs I/Os asynchronously.

Sure, but currently it all synchronous

> Or you use an event notification
> mechanism with multiple kernel threads performing I/Os synchronously.
> 
> Xen blktap has the same problem as stgt. IIRC, Xen mainline uses a
> kernel patch to add a proper event notification to AIO though redhat
> uses the same workaround as stgt instead of applying the kernel patch.
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