Re: stgt a new version of iscsi target?

[Vladislav Bolkhovitin <vst@vlnb.net>]

James Bottomley wrote:
> On Fri, 2005-12-09 at 18:29 +0300, Vladislav Bolkhovitin wrote:
> 
>>>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.
> 
> You do it the same way an array does:  the model for SPI is read command
> phase, disconnect, process command (i.e. set up areas) reconnect for
> data transfer.
> 
> map/unmap are really only necessary if you're emulating the data store,
> but it's a fairly cheap operation on linux: it just causes the creation
> of a vm_area.  If it's a pass through, you can use SG_IO to pull it in
> and the SG_IO like output to shoot it out again, effectively using a
> piece of user memory as a zero copy buffer.
> 
> Fragmentation isn't an issue because the I/O goes via sg lists , all
> that's needed is a bunch of pages.

OK, I see what you meant, thanks.

> 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.

Are you sure that there are no now or will be available in the nearest 
feature such (eg iSCSI) SCSI arrays with response time/latency so small 
that having 5 (five) context switches or more per command, some of which 
include map/unmap operations, will not increase the latency too much? I 
mean, eg NFS server, which originally was user space daemon and many 
people didn't want it in the kernel. Eventually, it's in. I don't see 
any fundamental difference between NFS server and SCSI target server, 
especially considering that SCSI is synchroneous protocol, where the 
latency matters a lot, and usually there is some FS over exported SCSI 
device, which not always does bulk data operations.

Vlad


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