RAID5 & RAID1
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Understanding How RAID Works

Many dedicated hosts implement RAID to achieve increased reliability by mirroring data on duplicate drives or by using error-recovery technology that allows on-the-fly data reconstruction.

Even if you won't be dealing with major tasks like RAID implementation, yet, understanding what it is and how it works won't hurt. Patterson, Gibson, and Katz developed the concept of RAID at the University of California at Berkeley in 1987. Their research paper was titled "A Case for Redundant Arrays of Inexpensive Disks (RAID)." The rationale behind RAID is to unite several small, inexpensive disks into an array of disk drives, offering performance levels exceeding that of a Single Large Expensive Drive (SLED). All disks in the array appear as a single logical storage unit, or a single disk, to the computer. Having this array of multiple disks allows parallelism and concurrency between operations to some extent, since the disk controller and the host processor are now capable of requesting or initiating read or write operations more swiftly.

RAID offers better load balancing than would otherwise be possible without it. Spreading a single file on various disks, all disks can work at the same time. Without this enhanced load balancing, some disks would end up doing the largest part of the work. By distributing the load, requests can be fulfilled more quickly because each disk has less to do. This allows each drive to work on a distinct read/write operation and maximize the number of simultaneous I/Os that the array can perform, thus increasing the transfer rate.

When looking at the specifications of your dedicated server's host, you will come across RAID1, RAID4, RAID5 etc. These are the different strategies of mirroring the drives; popular ones are RAID1 & 5.

RAID Level 1 provides disk mirroring - information is duplicated to both disks. In the event that one of your drives fails, the other will immediately continue to serve data. An additional feature with RAID1 is that, if one drive is jammed, the overflow requests will be handled by the second drive.

To maintain the mirroring, both disks write the same data. Therefore, RAID 1 offers no performance gains on write accesses. However, different read transactions can occur simultaneously and increase performance. RAID1 requires two drives, but you will actually lose the disk space of one of the drives, because it stores nothing but the duplicate content.

For a Web server application, RAID 5 provides the best performance and reliability. Read operations, such as those sustained by a Web server, are better with RAID 5 than with the RAID 1 counterpart. The efficiency of RAID 5 is based on the fact that it uses parity information instead of duplicating data. However, because parity information must be calculated for all drives with each write operation, write operations are not as efficient.

RAID Level 5 is N-1, meaning you lose the disk space of one drive in order to stripe data across the drives. A minimum of three drives is required for RAID5.

Other Sections In This Guide:
Server OS ... Before Anything Else, Hardware Issues, Managed VS Unmanaged Server, Considering A Virtual Private Server, Data Center & Server Connectivity Issues, Web Server Administration, DNS & BIND Overview, Remote Administration