Introduce the parameter to limit scsi timeout count (take 3)

Hi,

This is the updated patch to limit the scsi timeout count for the scsi-rc-fixes-2.6.

BACKGROUND
==========

A storage could break down in the way that a Logical Unit (LU) does not respond to scsi commands such as read/write, while the LU respond to scsi commands such as test unit ready (TUR).

I think that it is an ideal behavior that a storage returns an error for both TUR and R/W to the LU related to a failed disk unit so that OS can notice the failure of the LU. However, TUR and R/W are processed by different components in the storage. TUR is processed by a storage controller, and R/W is processed by a disk unit. When a disk unit takes time to process R/W, this type of error could happen in reality.

When this problem happens, the scsi-mid layer detects timeout for the LU. Then, scsi-mid layer tries to recover the error, and scsi-mid layer misunderstands that the LU has been recovered by the result of TUR. Therefore, the state of the device related to the LU is not changed to offline and user application can continue to issue I/Os to the LU. This may cause timeout errors repeatedly on the same LU, and application can not do proper actions quickly.

In addition, this issue seriously affects system boot time. During LU scanning in scsi-mid layer, read I/Os are issued to recognized LUs to get their partition table in check_partition(). Usually, many types of filesystems are registered to the kernel, and partition check is executed for each filesystem. This is a very long process because every read I/O ends up scsi timeout.

Moreover, scsi device scan is sequentially done, and other devices wait to be scanned. In some Linux distributions, boot processes go forward before valid devices are recognized, and system can not start correctly even if devices are fully redundant using mirroring.

SOLUTIONS
=========

Introduce the parameter to limit the maximum number of timeout count in scsi-mid layer. When the number of timeout count of a device reaches the maximum timeout count, the device is offlined. This parameter is configurable through sysfs.

In addition, to address the issue at kernel boot, a scsi module parameter, initparm, is added to set default values, timeout value and maximum timeout count. These values can be defined for devices identified by vender/model as scsi devinfo does.

PATCH SET
=========

1/2: Limit scsi timeout count per device 2/2: Interface to set default timeout related values

EXAMPLE
=======

* Limit a scsi timout count to 1

    # echo 1 > /sys/block/<sdX>/device/max_timeout_cnt
    # cat  /sys/block/<sdX>/device/max_timeout_cnt
    1

* Display a current timeout count

    # cat /sys/block/<sdX>/device/iotimeout_cnt
    0

* Load scsi module with a default scsi timeout(10s) and maximum timeout count (1) for HITACHI/DF600.

    # insmod scsi_mod.ko initparm="HITACHI:DF600:10:1"

* Show current default configurations.

    # cat /proc/scsi/initparm
    'HITACHI' 'DF600' timeout=10 max_timeout_cnt=1

PREVIOUS POSTS
==============

Introduce the parameter to limit scsi timeout count
http://www.spinics.net/lists/linux-scsi/msg36406.html

Introduce the parameter to limit scsi timeout count (take2)
http://www.spinics.net/lists/linux-scsi/msg36954.html

I appreciate your comments and suggestions.

Thanks,
---
Takahiro Yasui
Hitachi Computer Products (America), Inc.