Vladislav Bolkhovitin wrote:
Mike Christie wrote:
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 additi
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 miss anything?
Userspace only occurs for READ/WRITEs. So for REPORT_LUNS, TUR, etc
where performance is not a factor.
So, do you mean that READ/WRITE operations are (or will be) performed in
kernel space only? I don't see that from the code.
They are today. I think they have always been that way. Look again :)
From other side, stgt has not too much advantages over scst.
Hey, we just started and have not had too much time.
No offense, please, you wrote stgt and scst are the same, I wrote, they
aren't :-)
Yeah, I know. It was my mistake. If you read other mails, you would see
I mentioned that we do not have any type of eh and I make Tomo mess up
the serialization stuff becase I asked him to try to reuse some block
layer code.
If you read other mails, you would also see I reccomended scst to people
requesting something stable :)
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).
Simmer down :) If you had just gotton your code ready when christoph
asked a year ago we would never have had this problem and I would be
sending you patches to remove the scsi_request usage as we speak.
Christoph didn't ask me to do anything. Actually, nobody contacted and
What? Look at the review comments you got on the list when you first
posted scst. It asked you to take out the 2.4ish code out and clean it
up didn't it?
asked anything about that me when you with Christoph conceived stgt.
I do not think Christoph and I conceived stgt. Tomo and I did or
actually Tomo did the original code to push things to userspace. I just
bugged him about getting something in mainlin. We only asked Christoph
what we would should do to get things in mainline. After seeing what
open-iscsi did and the comments from Arjan about IET, we thought we
would have to follow a similar route as open-iscsi.
At this point I do not care what we do. I just want to get something in
mainline for everyone to use and so I can use it at home becuase I am
lazy and do not want to drive to work in the snow to test FC card
updates :) If we can come to some conclusions on some of these issues I
am sure Tomo and I would be happy to work together.
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