Re: [Scst-devel] Re: Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?

[Vladislav Bolkhovitin <vst@vlnb.net>]

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.
> 
> James
> 
> 
> 
> 
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