10-17-2012
Well, I am sure most SAN can be configured for mirroring, which is raid-2 or something like that. To achieve the most bang for your buck, you want the mirrors to be as far apart as possible, so something between the application host and short of the storage box needs to know where there are 2 so it can sidestep the dead side as well as splitting the query load and duplicating the churn. The farther upstream it is done, the greater the reliability, but too close can load the app server and communications, unless fiber has multicast write and anycast read. There may be several layers vying to mirror your storage, so pick wisely.
Mirroring got overshadowed a bit by raid, but has always had a query bandwidth advantage, with two devices handling query. Within the devices, there can be as much striping as in raid, so that is no different. When writing, there is no parity calculation and additional write time, just two immediate simultaneous writes. With raid in sequential striping mode, you write data to 1,2,3,4 and parity to 5, then data to 5,1,2,3 and parity to 4, and so on, so while reading is 5x spindle speed, writing is 4x. A mirrored pair trades space for bandwidth. Disk is cheap, and bandwidth is golden. Finally, it seems some raid systems seem to only get defects detected by staff when 2 adjacent devices fail, so often raid5 is either also mirrored or great downtime, data loss and partial restore pain is experienced.
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LEARN ABOUT REDHAT
raidstart
raidstart(8) System Manager's Manual raidstart(8)
NAME
raidstart, raidstop, - command set to manage md devices.
SYNOPSIS
raidstart [options] <raiddevice>*
raidstop [options] <raiddevice>*
DESCRIPTION
RAID devices are virtual devices created from two or more real block devices. This allows multiple disks to be combined into a single
filesystem, possibly with automated backup and recovery. Linux RAID devices are implemented through the md device driver.
If you're using the /proc filesystem, /proc/mdstat gives you informations about md devices status.
Currently, Linux supports linear md devices, RAID0 (striping), RAID1 (mirrroring), and RAID4 and RAID5. For information on the various lev-
els of RAID, check out:
http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
for new releases of the RAID driver check out:
ftp://ftp.fi.kernel.org/pub/linux/daemons/raid/alpha
Avaible commands are :
mkraid : configures (creates) md (RAID) devices in the kernel, banding multiple devices into one.
raidstart : activates (starts) an existing 'persistent' md device
raid0run : activates old nonpersistent RAID0/LINEAR md devices
raidstop : turns off an md device, and unconfigures (stops) it
By default, a systems RAID configuration is kept in /etc/raidtab, which can configure multiple RAID devices.
All of these tools work similiarly. If -a (or --all) is specified, the specified operation is performed on all of the RAID devices men-
tioned in the configuration file. Otherwise, one or more RAID devices must be specified on the command line. For example:
raid0run -a
Starts all of the 'old' RAID0 RAID devices specified in /etc/raidtab. If only /dev/md1 should be started, the following command should be
used instead:
raidstart /dev/md1
OPTIONS
-a, --all
Apply the command to all of the configurations specified in the config file.
-c, --configfile filename
Use filename as the configuration file (/etc/raidtab is used by default).
-h, --help
Displays a short usage message, then exits.
-V, --version
Displays a short version message, then exits.
NOTES
The raidtools are derived from the md-tools and raidtools packages, which were originally written by Marc Zyngier, Miguel de Icaza, Gadi
Oxman, Bradley Ward Allen, and Ingo Molnar.
BUGS
no known bugs.
SEE ALSO
raidtab(5), raid0run(8), raidstop(8), mkraid(8)
raidstart(8)