[Iscsitarget-devel] Re: [Scst-devel] Re: Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?
vst at vlnb.net
Fri Dec 9 16:29:27 CET 2005
James Bottomley wrote:
> On Thu, 2005-12-08 at 21:46 +0300, Vladislav Bolkhovitin wrote:
>> - Scst has some performance advantages over stgt, at least, on
>>hardware targets, because it allows internal handling in SIRQ context as
>>well as doesn't need user space program, so it eliminates additional
>>context switches (at least 3 per command for WRITEs and 2 per command
>>for reads plus switches to user space daemon, probably, 2 per command).
>>5 context switches per command looks too much for me, especially
>>considering how little work is done on each context. It means ~15000
>>CS/sec on regular 200Mb/sec link with 64K block size. Additionally,
>>kernel only commands execution allows direct access to page cache, which
>>is GREAT performance improvement, because in architecture with user
>>space daemon the data should be copied several times between kernel and
>>user space. Or, do I miimpss anything?
> I do have to say that I consider operation in interrupt context (or even
> kernel context) to be a disadvantage. Compared with the response times
> that most arrays have to SCSI commands, the kernel context switch time
> isn't that significant.
> Additionally, it's perfectly possible for all of this to be done zero
> copy on the data. A user space target mmaps the data on its storage
> device and then does a SG_IO type scatter gather user virtual region
> pass to the underlying target infrastructure. We already have this
> demonstrated in the SG_IO path, someone just needs to come up with the
> correct implementation for a target path.
I'm not completely understand how it will work. Consider, there are
READ/WRITE commands with random data sizes arrive from an initiator. Are
you going to do map/unmap for each command individually or alloc data
buffers for commands from a premapped area and live with possible its
fragmentation? If map/unmap individually, then I should say that those
are very expensive operations.
> The great advantage of doing SCSI state machines in user space is that
> you can prototype anything you want, and user space has much better
> state machine implementation (and debugging) tools available than the
> kernel does.
> This SF.net email is sponsored by: Splunk Inc. Do you grep through log files
> for problems? Stop! Download the new AJAX search engine that makes
> searching your log files as easy as surfing the web. DOWNLOAD SPLUNK!
> Scst-devel mailing list
> Scst-devel at lists.sourceforge.net
This SF.net email is sponsored by: Splunk Inc. Do you grep through log files
for problems? Stop! Download the new AJAX search engine that makes
searching your log files as easy as surfing the web. DOWNLOAD SPLUNK!
Iscsitarget-devel mailing list
Iscsitarget-devel at lists.sourceforge.net
More information about the stgt