[Stgt-devel] Performance of SCST versus STGT

[FUJITA Tomonori]

On Tue, 22 Jan 2008 14:33:13 +0300
Vladislav Bolkhovitin <vst at 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. 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.