Adding in Fault Tolerance for External Storage Systems

When implementing centralized storage solutions, you are often placing a large number of very important eggs into a single basket . SAN and NAS manufacturers understand this and have spent a lot of research and development dollars on building in Fault Tolerance into their offerings. There are many options available to the end user ; some of the fault tolerance options are as follows :

• RAID Configurations ” RAID levels 0+1 and 5 are most common. RAID level 6 offers the ability to lose two drives at a time and not lose data.

• Triple Mirroring ” This enables you to snap off a mirror so that data becomes static for purposes of backup. Meanwhile the system still has mirrored drives for fault tolerance. This is most commonly used with databases.

• Log Shipping ” Most SAN and NAS devices can copy log files in near real time to another SAN or NAS so that databases can be copied regularly and log files can be kept in synch remotely.

• Geographic Mirroring ” SAN and NAS devices offer in-band and out-of- band options for mirroring data across wide distances. Whereas SCSI has a 25- foot limitation, Fiber Channel can locate a device up to 1,000km away.

• Snapshotting ” By flagging disk blocks as being part of a particular version of a file and writing changes to that file on new blocks, a NAS or SAN device can take a snapshot of what the data looked like at a point in time. This enables a user to roll back a file to a previous version. It also enables you to roll back an entire system to a point in time almost instantly.

• Clustering ” NAS devices that use heads to serve data offer dual heads so that if one fails, the other will continue to serve data from the disks.

• Redundant power systems ” Any good SAN or NAS will offer multiple power supplies to protect against failure. Take advantage of the separate power supplies by attaching them to separate electrical circuits.

• Redundant back-planes ” Many NAS and SAN devices will offer redundant back-planes to protect against hardware failure.

• Hot Standby Drives ” By having unused drives available in the chassis, the device can replace a failed disk instantly with one that is already present and ready for use. Be sure to monitor the SAN or NAS device to see if a disk has failed and been replaced . It can be easy to miss because there would be no interruption to service.

RAID 5 Is Not Recommended

RAID 5 is not recommended for any application that performs write transactions more than about 15% of the time. This is due to the fact that each write transaction requires reading multiple disks and recalculating and writing of parity bits.