Raid levels

RAID levels


RAID-0

RAID-0 uses disk striping; that is, it writes data across multiple hard disk partitions in what is called A stripe set. This can greatly improve speed because multiple hard disks are working at the same time. You can implement RAID-0 through the use of Windows Server software or third-party hardware. Although RAID-0 gives you the best speed, it does not provide any fault-tolerance. If one of the hard disks in the stripe set is damaged, you lose all of your data. Because of the lack of faulttolerance, Microsoft doesn’t recommend storing any of your SQL Server data on RAID-0 volumes.



RAID-1

RAID-1 uses disk mirroring. Disk mirroring actually writes your information to disk twice — once to the primary file and once to the mirror. This gives you excellent fault-tolerance, but it is fairly slow, because you must write to disk twice. Windows Server allows you to implement RAID-1 with a single controller, or you can use a controller for each drive in the mirror, commonly referred to as disk duplexing. This is the recommended place for storing your transaction logs because RAID-1 gives fast sequential write speed (writing data in sequence on the disk rather than jumping from one empty spot to the next), a requirement for transaction logs.



RAID-5

RAID-5—striping with parity — writes data to the hard disk in stripe sets. Parity checksums will be written across all disks in the stripe set. This gives you excellent fault-tolerance as well as excellent speed with a reasonable amount of overhead. You can use the parity checksums to re-create information lost if a single disk in the stripe set fails. If more than one disk in the stripe set fails, however, you will lose all your data. Although Windows Server supports RAID-5 in a software implementation, a hardware implementation is faster and more reliable, and we suggest you use it if you can afford it. Microsoft recommends storing your data files on this type of RAID because data files require fast read speed as opposed to transaction logs, which need fast write speed.



RAID-10

You should use RAID-10 (sometimes referred to as RAID 0+1) in mission-critical systems that require 24/7 uptime and the fastest possible access. RAID-10 implements striping with parity as in RAID-5 and then mirrors the stripe sets. So, you get the incredible speed and faulttolerance, but RAID-10 has a drawback. With this type of RAID you get the added expense of using more than twice the disk space of RAID-1. Then again, we are talking about a situation that can afford no SQL Server downtime