[stgt] [PATCH 1/1] null backing store

[FUJITA Tomonori]

On Thu, 28 Aug 2008 16:39:22 +0300
"Alexander Nezhinsky" <nezhinsky at gmail.com> wrote:

> Sorry, I missed the list in my previous reply...
> 
> On Thu, Aug 28, 2008 at 2:09 AM, FUJITA Tomonori
> <fujita.tomonori at lab.ntt.co.jp> wrote:
> > On Wed, 27 Aug 2008 20:22:42 +0300
> > "Alexander Nezhinsky" <nezhinsky at gmail.com> wrote:
> >> On Wed, Aug 27, 2008 at 8:01 PM, FUJITA Tomonori
> >> <fujita.tomonori at lab.ntt.co.jp> wrote:
> >>
> >> > Thanks. But you can do faster. This patch incurs the thread and pipe
> >> > notification overheads. You don't need bs_thread stuff. See bs_aio.c
> >>
> >> Sure. I intended to do it that way later. I'm going to send some performance
> >> results over ISER in a couple of days, so i first used this
> >> straightforward implementation.
> >
> > Nice.
> >
> >> Moreover, it is also interesting to measure *with* the thread'n'pipe
> >> overhead because
> >> this way you only exclude I/O. It can be later compared with a version
> >> without threads
> >> to assess their impact.
> >
> > Yeah, the results should be interesting because we plan to remove the
> > thread mechanism (well, we need to wait for the AIO support in Linux
> > kernel).
> >
> >> > After that, you can do more faster. You can use sendfile (send data
> >> > without memory copy) though you need some changes to the core code.
> >>
> >> How is sendfile() applicable to this null i/o backing store? I am not
> >> sending anything
> >> to I/O anyway, so there is no memory copy to spare.
> >
> > You send data to an initiator without doing disk I/Os.
> >
> > Anyway, you are an iSER guy, so you can forget sendfile stuff.
> 
> You mean sendfile /dev/zero --> socket or socket -->  /dev/null in case of
> iSCSI/TCP, don't you?

I'm not sure if sendfile works with /dev/zero. You can use any file
because the null backing store just sends bogus data.

stgt allocates a buffer and sends bogus data in the buffer for READ
commands with the null backing store. stgt can do sendfile with a file
instead of allocating a buffer and sending the data in it.
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