I am sending this again as the initial mail did not make it to the list.
Hi All,
I thought a lot about whether I should write to mailing list about this
or not. I have requested help from the list before to complete this. May
be I am too crazy to develop something like this. When I think more and
more about this, it will be really useful, mainly for educational
purposes and it could be useful in other environments.
I like to know everyone's opinion on this. Please let me know your
thoughts and suggestions and its inclusion in the main line Linux kernel.
I have developed a Virtual HBA (Around ~600 lines of code). It is a
kernel module and works very well with 2.6.9 kernel. Basically it
registers a HBA (LLD) to the SCSI Mid layer. It implements a linked list
with all the SCSI commands queued from Mid layer. I registered a
character device driver. I ship out the SCSI commands off to user space
via this character device. There is a user space application monitoring
this character device looking for SCSI commands, process it and sends it
back to the mid layer through this character interface. At the moment,
the interface to the Virtual HBA inside the kernel is through read/write
and ioctls to the character device. I am trying to get rid of reading
and writing of SCSI request_buffer through character device and use
memmap/splice to avoid copying kernel buffer to user space(I am facing
some challenges as the request_buffer is not a linear buffer, but a
scatter gather buffer).
As I mentioned above, the HBA only transports the commands out side the
kernel. But the real job is done by the user space tool. Now there is
end less opportunity for this user space tool. It can impliment iSCSI,
NDMP, local files, shared files across the network, etc. People can even
write iSCSI initiator using Java! The advantage is that the main
transport is done by user space application. Currently i have
implemented a small scsi engine and it presents a file as a SCSI disk to
the kernel. It works great.
The Kernel driver is ready. It will need some modifications (Like
implimenting memap to avoid copying, integrating with sysfs/procfs,
etc). I am thinking in different directions on what I should include in
the driver. May be you guys can direct me in right direction. If you
think, there is not much potential for this in the Linux kernel, I will
stop working on this now as I already achieved what I wanted (Presenting
a file as a SCSI disk).
Please let me know your thoughts and suggestions.
Background:
-------------
I wanted to make a NDMP (A protocol used to manage tapes and perform
backup/resotre on NAS heads) initiator for Linux (Just out of my
interest).. I was not very good with doing stuff inside the kernel.
First my idea was to modify the Cisco iSCSI initiator and make it for
NDMP. But i found, it is too much of work + I already had a user space
implimentation of NDMP procedures. So I thought of making a virtual HBA
and expose the SCSI mid-layer to a user space application.
Thanks
Aboo
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