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

Mike Christie michaelc at
Sat Dec 10 02:15:53 CET 2005

Mike Christie wrote:
>>>> Actually, we would greatly appreciate if Mike or Christoph will tell 
>>>> us what is so wrong in scst on their opinion, so they started they 
>>>> own new project. (I'm not considering motivation like "I want to 
>>>> make my own in any case" seriously). Is scst too complicated? Do you 
>>>> think stgt will be simpler with all those features implemented?
>>> Didn't we go over this? To get SCST ready for mainline we would have 
>>> had a large cleanup effort. I started this and sent you a patch to 
>>> begin the cleanup. In the end some of the scsi people liked the idea 
>>> of throwing the non-read/write command to userspace and to do this we 
>>> just decided to start over but I have been cutting and pasting your 
>>> code and cleaning it up as I add more stuff.
>> The patches that I've seen were just pretty mechanic cleanups and 
>> renames, which could be done in a half of hour time and which I'm going 
> Yeah it was the beginning of the easy work. I did not mean that as an 
> example of evertthing. I thought you would remember when we discussed 
> this on linux-scsi before.
>> to do in any case before preparing the patch. So, that reason doesn't 
>> look convincing for me to throw away a big chunk of working code. 
>> Doing so you delayed SCSI targets development for at least to a year-two, 
> Hey, looked how long it took iscsi to get in becuase we wasted so much 
> time cleaning up iscsi-sfnet :)
>> because there are too much features for you to implement in stgt, 
>> which are already working and useful in scst.
> Well, there was more when you asked on linux-scsi. You have other things 
> like refcouting (we only are adding that in today, but we do get 
> references to the scsi_host when we access the qla2xxx ha at least). If 
> someone ripps out a qla2xxx card you will oops.
> We also did not want to hook in as a SCSI ULD interface becuase we did 
> not want to worry about what happens when poeple start using 
> usb-mass-storage for targets and LUNs. Look how many times we see Alan 
> Stern pop up for just the initiator side :) And to be honest DM would do 
> a lot of what tgt and scst want as far as giving us a reference to the 
> devive we want to use as a LUN and handling all the setup work so we 
> probably both messed up there :(
>>  From other side, if you look on scst closely you will see that:
>> - The user space gate could be easily and clearly done using existing 
>> dev handler's hooks
> Yeah and the problem is that we just do not believe those are 
> implemented in the correct place. We do not like class interface SCSI 
> hook in, when we can do the same thing from userspace.
>> - There is nothing in current SCST that could be moved in user space 
>> without sacrificing performance. Neither task management, nor UA 
>> processing, nor RESERVE/RELEASE emulation. Otherwise, you will have to 
>> pass *all* the commands to the user space to check allowed this 
>> command for processing or it shall be returned with an error.
> For non READ/WRITE, we are ok with that performance drops. Even for 
> doing READ/WRITEs in the kernel from interrupt context, we were going to 
> run from a softirq, but we thought allocating the whole command with 
> GFP_ATOMIC would not be liked so we pass it to the thread. And for when 
> we do pass through (using elv_add_request/blk_rq_execute_nowait), we can 
> do it in just the context swith needed for the memory allocation. But to 
> do GFP_ATOMOIC softirq or hw irq would not be a problem, although I do 
> not think we want to submit IO from the hw interrupt incase the queue 
> gets unplugged at the same time :)
> For non READ/WRITEs look how far open-iscsi went. And from James's 
> reply, you see that he thinks READs and WRITEs can go to userspace too, 
> so you know this is an uphill battle to get everything in the kernel.
>  SCST core is just
>> about 7500 lines of code. Is it too much?
> Ask the people that have to review the code? :) After sfnet, I learned 
> that it is sometimes best to get the basics in the kernel first so we do 
> not burn out the christoph robot :) I think part of this stems from the 
> fact that I touched pretty much every line in that driver to clean it up 
> and it took me about a year. And while I was beginning to clean up scst 
> I began to remember sfnet.
> But there are other cleanups like moving some of the state to per 
> target, cleaningup the scattlist allocation code and moving it to 
> scsi-ml so the SCSI ULDs can use them and convert them. There is also 
> thing like converting to the right APIs for 2.6 (rm kernel_thread, rm 
> scsi_request, rm proc, fixup class interface refcouting problems, fixup 
> scsi_device lack of refcounting usage, etc).

Oh yeah I think the other major issue at least I had with scst was that 
it was scsi specific and we wanted try and seperate things so if drivers 
like IET and vscsi are allowed then we could also do other drivers like 
a ATA over ethernet target driver or allow any other target driver that 
wanted to to hook in. I think you noted that we were spererating some 
protocol specific things as a distadvantage or mentioned it for some 
reason but I am not completely sure why and we may not agree on that 
issue too.

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