A small but hopefully useful set of SCSI commands are supported along with some crude error checking together with support for autosense (and a REQUEST SENSE command following a status of CHECK CONDITION will also fetch the sense buffer). The number of simulated devices and the shared RAM size for storage can be given as module parameters or boot time options if the scsi_debug driver is built into the kernel.
One application of this driver is to test out Richard Gooch's "sd-many" SCSI disk device patch. The standard kernel (at least to lk 2.4.10) can only support 128 SCSI disk devices. Some large users are finding that this is a restriction and Richard (who is also responsible for devfs) has provided a patch to and is looking for testers. To date I have used this driver to simulate over 300 SCSI disk devices on relatively modest hardware with not much RAM (by modern standards). The "sd-many" patch can theoretically support over 2000 SCSI disk devices.
The original scsi_debug driver found in the lk 2.4 series was written by Eric Youngdale. Eric used it (and perhaps still does) to test the "new" error handling introduced originally in the lk 2.2 series and extended in the lk 2.4 series. There are also hooks through to the user space where nasty error situations could be tested without compromising the kernel. See Eric's site for more information.
If the function of this driver is unclear then the diagram in this accompanying
page may help.
Driver options can be seen with this command: modinfo -p scsi_debug . The "scsi_debug_num_devs" option allows the number of devices to be specified. It should be 1 or greater. Unless Richard Gooch's "many-sd" patch is in place, the upper limit is 128 (less any real SCSI disk devices already present). The default number of devices is 1. The "scsi_debug_dev_size_mb" option allows the size of the shared RAM used to simulate storage to be specified. The unit is megabytes and the default value is 8 (megabytes). The RAM reserved for storage is initialized to zeoes when the driver is loaded.
"scsi_debug_opts" takes a decimal number as an argument which is the "or" of several flags. This is new in 1.58 (soon to be in 0.58). The flags supported are:
"scsi_debug_every_nth" takes a decimal number as an argument. When this number is greater than zero and the "every_nth" bit is set in "scsi_debug_opts", then incoming commands are counted and when <n> is reached then the associated command is ignored. This will cause a timeout and associated error processing. Once the command count reaches <n> then it is reset to zero. This means setting "scsi_debug_every_nth" to 1 and "scsi_debug_opts" to 4 will cause every command to be ignored (and hence a timeout). If "scsi_debug_every_nth" is not given it is defaulted to 100, however this will not cause timeouts until the "every_nth" bit is set in "scsi_debug_opts" (which can also be set by writing to /proc/scsi/scsi_debug/<host_number> ).
"scsi_debug_delay" is found in version 0.61 (and beyond) for the lk 2.4 series and version 1.61 (and beyond) for the lk 2.5 series. It is the number of jiffies (10 milliseconds on i386) that the driver will delay responses. The default is 1 jiffy. Setting this option to 0 (or negative) will cause the response to be sent back to the mid level before the request function is completed. Both delayed and immediate responses are permitted however delayed responses are more realistic. For delayed responses, a kernel timer is used. [Real adapters would generate an interrupt when the response was ready (i.e. the command had completed).]
The supported SCSI commands are:
Modern SCSI devices use vital product page 0x83 for identification.
This driver yields both "T10 vendor identification" and "NAA" descriptors.
The former yields an ASCII string like "Linux scsi_debug
4000" where the "4000" is the ((host_no + 1) * 1000) + target_id). In this
case "4000" corresponds to host_no==3 and target_id==0 . The "NAA" descriptor
is an 8 byte binary value that looks
like this hex sequence: "51 23 45 60 00 00 0f a0" where the IEEE company id is 0x123456 (fake) and the vendor specific identifier in the least signficant bytes is 4000 in hex. [The "4000" is derived the same way for both descriptors.]
Reads and writes are atomic (i.e. a write from one scsi_debug device will not interrupt (split) a read from another scsi_debug device.
Other commands can easily be added so if you have some suggestions,
then please contact me.
Since the scsi_debug driver manipulates "ram" rather than a HBA attached
to the PCI bus, its devices appear under the /driverfs/root/scsi<n>
directory (where <n> is the host number) rather than a directory like
The device "name" for driverfs is obtained from the SCSI INQUIRY vital
product data page 0x83 (naa 0x5 descriptor). One of my fake scsi_debug
disks gives this output:
$ cat /driverfs/root/scsi3/3:0:0:0/name
Each scsi_debug disk has a distinct "name". [Since they contain the
same data perhaps they shouldn't.]
To build it, CONFIG_SCSI_DEBUG needs to be selected in the kernel configuration.
It is the last SCSI low level driver listed. [You may also need to select
CONFIG_EXPERIMENTAL in the code maturity level section otherwise scsi_debug
isn't listed.] Assuming it is being built as a module then the following
would be appropriate:
$ cd /usr/src/linux
$ make modules
$ make modules_install
The driver could then be started with:
$ modprobe scsi_debug scsi_debug_num_devs=12
to simulate 12 scsi disk devices, each sharing the same 8 MB or RAM (i.e. the default RAM size).
Here is a table of the versions of scsi_debug available for the lk 2.4
(production) and 2.5 (development) series. Where the version appears in
bold type then the kernel contains that version of the scsi_debug driver.
|Linux version||scsi_debug version||tarball||notes|
|lk 2.4.18||0.57||scsi_debug_057.tgz||first rework|
|lk 2.4.19-pre*||0.58||scsi_debug_058.tgz||more inquiry+mode_sense, opts, vmalloc|
|2.4 experimental||0.61||scsi_debug_061.tgz||scsi_debug_every_nth, delay [rework]|
|lk 2.5.3||1.57||scsi_debug_157.tgz||closely related to version 0.57|
|lk 2.5.15||1.59||scsi_debug_158.tgz||like 0.58, tracking lk 2.5 series changes|
|2.5 experimental||1.61||scsi_debug_161.tgz||scsi_debug_every_nth, delay [rework] lk 2.5.28|
$ cat /proc/scsi/scsi
Host: scsi1 Channel: 00 Id: 01 Lun: 00
Vendor: IBM Model: DNES-309170W Rev: SA30
Type: Direct-Access ANSI SCSI revision: 03
Host: scsi5 Channel: 00 Id: 00 Lun: 00
Vendor: Linux Model: scsi_debug Rev: 0002
Type: Direct-Access ANSI SCSI revision: 02
In this case there is one real SCSI device (an IBM disk) and one scsi_debug pseudo disk. The fdisk /dev/sdb command can now be used to partition the disk. Assuming one ext2 partition is allocated the whole pseudo disk (8 MB in this case) then the mke2fs /dev/sdb1 command can be used to make an ext2 file system. Now /dev/sdb1 can be mounted and treated like a normal file system. Naturally when the power is turned off anything stored in /dev/sdb1 will be forgotten.
Rather than mounting the pseudo disk, the sg3_utils package (see the main page) could be used to carry out various test on it.
Information about the scsi_debug driver version, its current parameters and some other data can be found in the normal way the Linux SCSI lower level drivers provide data:
# cat /proc/scsi/scsi_debug/3
scsi_debug adapter driver, Version: 1.61 (20020719)
num_devs=1, shared (ram) size=8 MB, opts=0x0, every_nth=100(curr:0)
sector_size=512 bytes, cylinders=64, heads=8, sectors=32, delay=1
number of aborts=0, device_reset=0, bus_resets=0, host_resets=0
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Doug Gilbert (firstname.lastname@example.org)
Last updated: 25th July 2002