sg_reset - sends SCSI device, bus or host reset; or checks reset state


sg_reset [-b] [-d] [-h] [-V] DEVICE


The sg_reset utility with no options (other than DEVICE) reports on the reset state (e.g. if a reset is underway) of DEVICE. When given a -d, -b, or -h option it requests a device, bus or host reset respectively.

In the linux kernel 2.6 series this utility can be called on sd, sr (cd/dvd), st or sg device nodes; if the user has appropriate permissions.

In the linux kernel 2.4 series support for this utility first appeared in lk 2.4.19 and could only be called on sg device nodes. Various vendors made this capability available in their kernels prior to lk 2.4.19.


-b attempt a SCSI bus reset. This would normally be tried if the device reset (i.e. option -d) was not successful.
-d attempt a SCSI device reset. If the device seems stuck, this is the first reset that should be tried. This assumes the linux scsi mid level error handler is not already in the process of resetting DEVICE.
-h attempt a host adapter reset. This would normally be tried if both device reset (i.e. option -d) and bus reset (i.e. option -b) were not successful.
-V prints the version string then exits.


The error recovery code within the linux kernel when faced with SCSI commands timing out and no response from the device (LU), first tries a device reset and if that is not successful tries a bus reset. If that is not successful it tries a host reset. Users of this utility should check whether such a recovery is already underway before trying to reset with this utility. The "device,bus,host" order is also recommended (i.e. first start with the smallest hammer).

SAM-4 defines a hard reset (which includes a device power on and a transport reset), a logical unit reset and a I_T nexus reset. A LU reset and an I_T nexus reset can be requested via task management function (and support for LU reset is mandatory). In Linux the SCSI subsystem leaves it up to the low level drivers as to whether a "device reset" is only for the addressed LU or all the LUs in the device that contains the addressed LU. The "bus reset" is a transport reset. A "host reset" attempts to re-initialize the HBA that the request passes through on route to the DEVICE. Obviously a "host reset" and a "bus reset" can cause collateral damage.

This utility does not allow individual SCSI commands (or tasks as they are called in SAM-4) to be aborted. SAM-4 defines ABORT TASK and ABORT TASK SET task management functions for that.


Written by Douglas Gilbert.


Copyright © 1999-2006 Douglas Gilbert This software is distributed under the GPL version 2. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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