configure SCT ERC on FreeBSD startup

Synopsis

/etc/rc.d:

# Configure read/write timeouts on disks in redundant RAID so
# if read/write error occurs, array performance is not severely
# degraded by disks trying to read/write data for too long
scterc_enable="YES"
scterc_disks="ada0 ada1"  # disks to configure
scterc_read_timeout="70"  # 7 seconds
scterc_write_timeout="70" # 7 seconds

About SCT ERC

Modern hard drives allow to set the amount of time a hard disk is allowed to spend recovering from a read or write error. This feature is called ERC (error recovery control, usually in Seagate), TLER (time-limited error recovery, usually on Western Digital) or CCLT (command completion time limit, usually on Samsung or Hitachi).

Desktop disks usually have this feature disabled by default and don't limit time of read/write request, so they have highest chances of recovering from errors and not losing data.

Disks designed for RAID usage, in contrast, have this feature enabled by default and set a small timeout (usually 7 seconds), as in redundant environment data may be quickly accessed from another device, and a single disk should not delay requests for too long, degrading array performance.

However, it is common now that even desktop disks are used in redundant configurations, and most modern disks, though still having ERC disabled by default, allow enabling it and setting a timeout, so it is recommended to enable this setting to not have system performance severely degraded when one of the drives start to experience read/write errors.

The ERC setting, however, does not survive reboot and requires updating it each time the system boots, and that's what this script does.

Viewing and setting SCT ERC by hand

You can view and change SCT ERC with smartctl from smartmontools package:

To view:

% smartctl -l scterc /dev/ada0 
smartctl 6.2 2013-07-26 r3841 [FreeBSD 10.0-RELEASE amd64] (local build)
Copyright (C) 2002-13, Bruce Allen, Christian Franke, www.smartmontools.org

SCT Error Recovery Control set to:
           Read: Disabled
          Write: Disabled

To set:

% smartctl -l scterc,70,70 /dev/ada0
smartctl 6.2 2013-07-26 r3841 [FreeBSD 10.0-RELEASE amd64] (local build)
Copyright (C) 2002-13, Bruce Allen, Christian Franke, www.smartmontools.org

SCT Error Recovery Control set to:
           Read:     70 (7.0 seconds)
          Write:     70 (7.0 seconds)

Setting SCT ERC by this script

After installing this script by hand or with port, you should add some lines to /etc/rc.conf to tell it which disks to configure and which timeouts to set.

For simple configurations, there are 4 rc.conf variables to set:

• scterc_enable set to YES to enable the script
• scterc_disks specify space separated list of disks to configure
• scterc_read_timeout specify read timeout in tenths (1/10) of second, default is 70 (7.0 seconds)
• scterc_write_timeout specify write timeout in tenths (1/10) of second, default is 70 (7.0 seconds)

For more complex configurations when you have multiple disk groups for which you want different settings:

• scterc_enable set to YES to enable the script
• scterc_groups specify space separated list of disk group names

Then, for each group, you may specify individual settings:

• scterc_{group}_disks specify space separated list of disks to configure
• scterc_{group}_read_timeout specify read timeout in tenths (1/10) of second, default is scterc_read_timeout value
• scterc_{group}_write_timeout specify write timeout in tenths (1/10) of second, default is scterc_read_timeout value

You may use scterc_disks and scterc_groups at the same time.

Example configuration

scterc_enable="YES"

scterc_disks="ada0 ada1"
scterc_read_timeout=100
scterc_write_timeout=100

scterc_groups="group1 group2"

scterc_group1_disks="ada2 ada3"
scterc_group1_read_timeout="120"
scterc_group1_write_timeout="120"

scterc_group2_disks="ada4 ada5"
scterc_group2_read_timeout="140"

This will configure read/write timeouts like this ada0:100/100, ada1:100,100, ada2:120/120, ada3:120/120, ada4:140/100, ada5:140/100.

Diagnostics

To check your configuration, you can run script by hand, after which it will print which drives it configure and which timeouts is sets:

$ /usr/local/etc/rc.d/scterc start
Setting SCT ERC on disks: ada0:100,100 ada1:100,100 ada2:120,120 ada3:120,120
ada4:140,100 ada5:140,100.
$

The same line will be printed on the system startup. In case of error, ERROR message will be printed instead of timeout values for each disk which has failed to configure:

Setting SCT ERC on disks: ada0:ERROR ada1:ERROR

if that happens, check your config and/or run smartctl by hand as specified above and check its output for errors.

See also

• smartctl(8) manual page
• Wikipedia article on ERC
• Article on SCT ERC (Russian)
• Your hard drive's documentation

License

CC0. This is just a simple script.

Author

• Dmitry Marakasov amdmi3@amdmi3.ru