scst/scst_local

SCST Local ...
Richard Sharpe, 30-Nov-2008

This is the SCST Local driver. Its function is to allow you to access devices
that are exported via SCST directly on the same Linux system that they are
exported from.

No assumptions are made in the code about the device types on the target, so
any device handlers that you load in SCST should be visible, including tapes
and so forth.

You can freely use any sg, sd, st, etc. devices imported from target,
except the following: you can't mount file systems or put swap on them.
This is a limitation of Linux memory/cache manager. See SCST README file
for details.

To build, simply issue 'make' in the scst-local directory.

Try 'modinfo scst_local' for a listing of module parameters so far.

NOTE! This is now part of scst and supports being built in the scst tree.

Here is how I have used it so far:

1. Load up scst:

  modprobe scst
  modprobe scst_vdisk

2. Create a virtual disk (or your own device handler):

  dd if=/dev/zero of=/some/path/vdisk1.img bs=16384 count=1000000
#  dd if=/dev/zero of=/some/path/vdisk2.img bs=16384 count=1000000
  echo "open vm_disk1 /some/path/vdisk1.img" > /proc/scsi_tgt/vdisk/vdisk
#  echo "open vm_disk2 /some/path/vdisk2.img" > /proc/scsi_tgt/vdisk/vdisk
  echo "add vm_disk1 0" > /proc/scsi_tgt/groups/Default/devices
#  echo "add vm_disk2 1" > /proc/scsi_tgt/groups/Default/devices

3. Load the scst_local driver:

  insmod scst_local
# insmod scst_local max_luns=8

4. Check what you have

   cat /proc/scsi/scsi
  Attached devices:
  Host: scsi0 Channel: 00 Id: 00 Lun: 00
    Vendor: ATA      Model: ST9320320AS      Rev: 0303
    Type:   Direct-Access                    ANSI  SCSI revision: 05
  Host: scsi4 Channel: 00 Id: 00 Lun: 00
    Vendor: TSSTcorp Model: CD/DVDW TS-L632D Rev: TO04
    Type:   CD-ROM                           ANSI  SCSI revision: 05
  Host: scsi7 Channel: 00 Id: 00 Lun: 00
    Vendor: SCST_FIO Model: vm_disk1         Rev:  101
    Type:   Direct-Access                    ANSI  SCSI revision: 04

5. Have fun.

Some of this was coded while in Santa Clara, some in Bangalore, and some in
Hyderabad. Noe doubt some will be coded on the way back to Santa Clara.

The code still has bugs, so if you encounter any, email me the fixes at:

   realrichardsharpe@gmail.com

I am thinking of renaming this to something more interesting.


Limitations
===========

Swapping on scst_local's devices and using a writable mmap over a file
on them are not supported due to possible OOM deadlock in the Linux
memory/cache manager. See SCST README for more details.


Note on performance
===================

Although this driver implemented in the most performance effective way,
including zero-copy passing data between SCSI/block subsystems and SCST,
in many cases it is NOT suited to measure performance as a NULL link.
For example, it is not suited for max IOPS measurements. This is because
for such cases not performance of the link between the target and
initiator is the bottleneck, but CPU or memory speed on the target or
initiator. For scst_local you have both initiator and target on the same
system, which means each your initiator and target are much less
CPU/memory powerful.


Change log
==========

V0.1 24-Sep-2008 (Hyderabad) Initial coding, pretty chatty and messy,
                             but worked.

V0.2 25-Sep-2008 (Hong Kong) Cleaned up the code a lot, reduced the log
			     chatter, fixed a bug where multiple LUNs did not
			     work. Also, added logging control. Tested with
			     five virtual disks. They all came up as /dev/sdb
			     through /dev/sdf and I could dd to them. Also
			     fixed a bug preventing multiple adapters.

V0.3 26-Sep-2008 (Santa Clara) Added back a copyright plus cleaned up some
			       unused functions and structures.

V0.4 5-Oct-2008 (Santa Clara) Changed name to scst_local as suggested, cleaned
			      up some unused variables (made them used) and
			      change allocation to a kmem_cache pool.

V0.5 5-Oct-2008 (Santa Clara) Added mgmt commands to handle dev reset and
			      aborts. Not sure if aborts works. Also corrected
			      the version info and renamed readme to README.

V0.6 7-Oct-2008 (Santa Clara) Removed some redundant code and made some
			      changes suggested by Vladislav.

V0.7 11-Oct-2008 (Santa Clara) Moved into the scst tree. Cleaned up some
			       unused functions, used TRACE macros etc.

V0.9 30-Nov-2008 (Mtn View) Cleaned up an additional problem with symbols not
			    being defined in older version of the kernel. Also
			    fixed some English and cleaned up this doc.