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Understanding RAID levels for storage systems

  • Sep 25 / 2008
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DBA Interview questions, dbDigger, Storage, System Administration

Understanding RAID levels for storage systems

As data is of primary importance for any organization. And DBAs pay utmost attention to make sure the reliability and performance of Relational data. SQL Server databases are configured on several levels of RAID. There are many RAID levels available on modern RAID controllers. Only a subset of these is most useful when configuring a Microsoft SQL Server. Each RAID level has a specific use and benefit. Using the wrong type of RAID level can not only hurt system performance, but also add more cost to your server configuration.
Experts recommend that you never use software arrays on a database server. Use of software arrays requires additional CPU power from Windows in order to calculate which physical disk the data is written to.
In hardware arrays, this overhead is offloaded to a physical PCI, PCIe or PCIx card within the computer (or within the SAN device), which has its own processor and software dedicated to this task.

RAID 1 – Mirror.
A RAID 1 array is most useful for high write files, such as the page file, transaction logs and tempdb database. A RAID 1 array takes two physical disks and creates an exact duplicate of the primary drive on the backup drive. There is no performance gain or loss when using a RAID 1 array. This array can survive a single drive failure without incurring any data loss.

RAID 5 – Redundant Stripe Set.
A RAID 5 array is most useful for high read files such as the database files (mdf and ndf files) and file shares. It is the most cost-effective, high-speed RAID configuration. With a RAID 5 array, there is a performance impact while writing data to the array because a parity bit must be calculated for each write operation performed. For read performance, the basic formula is (n-1)*o where n is the number of disks in the RAID 5 array and o is the number of I/O operations each disk can perform. Note: While this calculation is not perfectly accurate, it is generally considered close enough for most uses. A RAID 5 array can survive a single drive failure without incurring any data loss.

RAID 6 – Double Redundant Stripe Set.
Like a RAID 5 array, a RAID 6 array is most useful for high read files such as the database and file shares. With RAID 6, there is also a performance impact while writing data to the array because two parity bits must be calculated for each write operation performed. The same basic formula is used to calculate the potential performance of the drives (n-2)*o. A RAID 6 array can survive two drive failures without incurring any data loss.Because of the dual parity bits with RAID 6, it is more expensive to purchase than a RAID 5 array. However, RAID 6 offers a higher level of protection than RAID 5. When choosing between RAID 5 and RAID 6, consider the length of time to rebuild your array, potential loss of a second drive during that rebuild time, and cost.

RAID 10 – Mirrored Strip Sets.
A RAID 10 array is most useful for high read or high write operations. RAID 10 is extremely fast; however, it is also extremely expensive (compared to the other RAID levels available). In basic terms, a RAID 10 array is several RAID 1 arrays stripped together for performance. As with a RAID 1 array, as data is written to the active drive in the pair, it is also written to the secondary drive in the pair. A RAID 10 array can survive several drive failures so long as no two drives in a single pair are lost.

RAID 50 – Stripped RAID 5 Arrays.
A RAID 50 array is an extremely high-performing RAID array useful for very high-load databases. This type of array can typically only be done in a SAN environment. Two or more RAID 5 arrays are taken and stripped together and data is then written to the various RAID 5 arrays. While there is no redundancy between RAID 5 arrays, it’s unnecessary because the redundancy is handled within the RAID 5 arrays. A RAID 50 array can survive several drive failures so long as only a single drive per RAID 5 array fails.

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