Xen devel - Aug 2006 - [PATCH 0/6] SCSI frontend and backend drivers

This patchset includes an updated version of the SCSI frontend and
backend drivers.

The frontend and backend drivers exchange SCSI RDMA Protocol (SRP)
messages via a ring buffer. The backend driver sends SCSI commands to
the user-space daemon, which performs SCSI commands and I/O
operations. The backend driver uses VM_FOREIGN feature like the blktap
driver for zero-copy of data pages.

Like the blktap, the user-space daemon can enjoy the advantages of
user-space I/O, that is, it can export raw partitions, regular files,
network, etc to VMs, and use nice tricks like snapshot by using
metadata disk formats, QoS, etc. In addition, the user-space daemon
performs SCSI protocol so that it can use SCSI device virtualization
like Virtual Tape Library, which exports files as tape drives to VMs.

The backend driver exploits the SCSI target framework (tgt), which
aims to add target driver support in Linux SCSI mid layer. It has been
included in the -mm tree. This patchset includes a modified version of
tgt code.


- Instructions

This patchset is against cset 10426:840f33e54054.

`make world && make install` should work. The user-space daemon code
is at tools/tgtd directory.

Configure your VM config file to use the SCSI drivers. If you like to
export /dev/sdb1 as a logical unit to a VM, add the following line:

scsi = [ ''disk:/dev/sdb1,w'' ]

You must run the user-space daemon before starting domainUs
(otherwise Domain0 crashes).

/usr/sbin/tgtd -f -d8

The above command runs the daemon in the foreground. So run start VMs
with another terminal.

If everything goes well, a SCSI host shows up in your VM (DomainU).

clematis:/# cat /sys/class/scsi_host/host0/proc_name
scsifront

And you can see a disk driver:

clematis:/# ls /sys/class/scsi_device/0\:0\:0\:0/device/
block:sda       iocounterbits  queue_depth          scsi_level  vendor
bus             iodone_cnt     queue_type           state
delete          ioerr_cnt      rescan               timeout
device_blocked  iorequest_cnt  rev                  type
driver          model          scsi_device:0:0:0:0  uevent


- Status and TODO

I manage to do fdisk, dd with a virtual disk drive (on DomainU),
however, it''s still far from completion. You cannot even stop the
drivers (sorry, it makes VMs crash).

Now, the user-space daemon uses read/write system calls for I/O. Soon
I will add AIO support and compare the performances of SCSI, blktap,
and blkback to examine SCSI overhead.

The current configuration format need to be improved, e.g., it can
handle only one logical unit per host. VMs cannot share a logical
unit.

I will take some code from the blktap daemon like metadata disk
formats, etc. The tgt daemon and the blktap daemon will have lots of
common features, so maybe it would be better to merge them later on.


- For SCSI people

I''ve not implemented the SRP protocol fully yet. For example, without
a login command, the frontend driver sends SCSI commands. I will work
on this later on. Probably, something like scsi_transport_srp would be
nice because of other SRP drivers (InfiniBand, IBM pSeries).

Another issue is the limitation on the number of sg table (now 11)
because I put indirect buffer in a ring buffer (like IB srp initiator
driver). I will work on this too later on.

The patchset includes libsrp.c, but it''s just makeshift. I will
rewrite it completely.


o Linux SCSI target framework (tgt)
http://stgt.berlios.de/

o The old version of SCSI frontend and backend drivers
http://lists.xensource.com/archives/html/xen-devel/2006-06/msg00849.html



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