Fedora Linux

unused devices: <none>

Likewise, to add a hot spare to an existing array, simply add an extra drive:

# mdadm --add /dev/md0 /dev/sdh1
mdadm: added /dev/sdh1

Since hot spares are not used until another drive fails, it's a good idea to spin them down (stop the motors) to prolong their life. This command will program all of your drives to stop spinning after 15 minutes of inactivity (on most systems, only the hot spares will ever be idle for that length of time):

# hdparm -S 180 /dev/[sh]d[a-z]

Add this command to the end of the file /etc/rc.d/rc.local to ensure that it is executed every time the system is booted:

#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.
touch /var/lock/subsys/local
hdparm -S 180 /dev/[sh]d[a-z]

6.2.1.7. Monitoring drive health

Self-Monitoring, Analysis, and Reporting Technology (SMART) is built into most modern disk drives. It provides access to drive diagnostic and error information and failure prediction.

Fedora provides smartd for SMART disk monitoring. The configuration file /etc/ smartd.conf is configured by the Anaconda installer to monitor each drive present in the system and to report only imminent (within 24 hours) drive failure to the root email address:

/dev/hda -H -m root
/dev/hdb -H -m root
/dev/hdc -H -m root

(I've left out the many comment lines that are in this file.)

It is a good idea to change the email address to the same alias used for your RAID error reports:

/dev/hda -H -m raid-alert
/dev/hdb -H -m raid-alert
/dev/hdc -H -m raid-alert

If you add additional drives to the system, be sure to add additional entries to this file.

6.2.2. How Does It Work?

Fedora's RAID levels 4 and 5 use parity information to provide redundancy. Parity is calculated using the exclusive-OR function, as shown in Table 6-4.

Table 6-4. Parity calculation for two drives

Bit from drive A	Bit from drive B	Parity bit on drive C
0	0	0
0	1	1
1	0	1
1	1	0

Notice that the total number of 1 bits in each row is an even number. You can determine the contents of any column based on the values in the other two columns ( A = B XOR C and B = A XOR C ); in this way, the RAID system can determine the content of any one failed drive. This approach will work with any number of drives.

Parity calculations are performed using the CPU's vector instructions (MMX/3DNow/SSE/AltiVec) whenever possible. Even an old 400 MHz Celeron processor can calculate RAID 5 parity at a rate in excess of 2 GB per second.

RAID 6 uses a similar but more advanced error-correcting code (ECC) that takes two bits of data for each row. This code permits recovery from the failure of any two drives, but the calculations run about one-third slower than the parity calculations. In a high-performance context, it may be better to use RAID 5 with a hot spare instead of RAID 6; the protection will be almost as good and the performance will be slightly higher.

6.2.3. What About...