Thursday, November 20, 2014

RAID

RAID : Redundant Array of Inexpensive Disks.
It is used to provide fault tolerance to database servers.  There are six RAID levels from 0 - 5. RAIDs will allow you to treat multiple hard drives as a single volume on your computer.

For SQL Server RAID 1+0 is the best one. If it is expensive RIAD 1 or 1+0 for log files 5 for data files and 10 for tempdb.  RAID 5 is not suitable for log file drives because there are huse writes on the files. RAID 1+0 provides better write performance than any other RAID level providing . So always plays log files on RAID (1+0) disk which provides better performance from hardware failures and better write performance.
Performance may benefits if TEMPDB is place on RAID 1+0
RAID 0 (Disk Striping) :It improve the read and write performance by spreading the operations into multiple disks. It is similar to RAID 5 but it wont provides fault tolerance 
RAID 1 (Disk Mirroring) : Disk mirror provides a redundant, identical copy of a selected disk. All data written into the primary disk is written into the mirror disk. RAID 1 provides the fault tolerance and generally improves the read performance but degrades write performance
RAID 2  : It adds redundancy by using an error correction method that spreads across all disks. It is not as efficient as other RAID levels and not generally used.
RAID 3  (Byte Striping) : It uses the striping method as RAID2, but error correcting method requires only one disk, it uses one drive on the array to store parity data (Parity data is used by some RAID levels to achieve redundancy. If a drive in the array fails, remaining data on the other drives can be combined with the parity data (using the Boolean XOR function) to reconstruct the missing data)
RAID 4 This level uses striped data in much larger blocks or segments than RAID 2 or RAID 3. Like RAID 3, the error correction method requires only one disk for parity data. This feature keeps user data separate from error-correction data. RAID 4 is not as efficient as other RAID levels and is not generally used.
RAID 5 (Parity Striping) : It is also known as parity with striping, in this level strips the data into large blocks across the disk in array. It writes the parity across all the disks. Data redundancy is provided by parity information.  Read data transactions are very fast while write data transaction are somewhat slower.  In the event of a single drive failure, data can still be accessed. A minimum of three drives is required.  
RAID 1+0 (Mirror of Strips) :  It is also known as mirror with striping. It uses the stripped array of disks that are then mirrored to another identical set of stripped disks. For example, a striped array can be created by using five disks. The striped array of disks is then mirrored using another set of five striped disks. RAID 10 provides the performance benefits of disk striping with the disk redundancy of mirroring. RAID 10 provides the highest read-and-write performance of any one of the other RAID levels, but at the expense of using two times as many disks