What is RAID 1? RAID Mirroring

RAID 1 is a level of the data storage technology RAID (Redundant Array of Independent Disk). It was first introduced in the 1980s as Redundant Array of Inexpensive Disks, highlighting its comparative cheapness. Each version of RAID—there are five primary ones—are known as levels, and RAID 1 is the second level introduced (after RAID 0). RAID 1 uses a technique called disk mirroring to copy data to another disk and requires a minimum of two drives.

RAID 0 and RAID 1: Striping and Mirroring

Disk mirroring improves upon striping, the technology used for RAID 0. Striping spreads sections of data across disks, distributing file contents across the entire array. Read and write operations are fast on striped disks because all the drives can be read at once, since the data exists in different places in the array. RAID 0 doesn’t provide protection against data failure; the data is not backed up, and if one disk fails, the content on it doesn’t exist elsewhere in the array.

RAID 1 doesn’t use striping at all. It mirrors disks in sets of two, so that two copies of a file exist. Mirrored disk arrays must always have even numbers. The word redundancy used in RAID describes repetition for the sake of protecting data. However, because RAID 1 keeps exact copies of data, corruption within one file will result in the same corruption on the corresponding drive.

A RAID controller is a piece of hardware that creates the redundancy between disks, used to configure a RAID array’s setup. Disks should be the same size and capacity. When managing a new disk array, a RAID user will select the mirror option for a new volume to set up RAID 1.

RAID 1 effectively removes half of the storage capacity of an array. Although each disk does have a specified capacity (say 500 GB), the data is mirrored on another disk, so only the same files can be stored on both drives. This is one of the biggest complaints that storage experts have about disk mirroring.

RAID 1 and RAID 0 have been combined in RAID 1+0, commonly known as RAID 10, which uses both mirroring and striping. Using striping with mirroring adds read speed that RAID 1 otherwise lacks.

Disadvantages of RAID 1

  • RAID 1 is not the best provider of redundancy for very large servers that are storing terabytes of important data. It has slower write speeds than RAID 0, because data has to be written to mirrored drives both times.
  • RAID 1 is commonly used as a backup solution, but it’s not reliable as the only backup method for data. It doesn’t protect data from power surges or other physical damage (since it’s not in the cloud or on another server). And as previously mentioned, RAID 1 doesn’t protect data from corruption; if part of a file is corrupted, the same corruption will be exactly mirrored on the other drive.
  • It offers half the capacity of true data storage because half is used for copies.

RAID 1 for SSDs

Solid-state drives are a much faster non-volatile data storage solution than hard drives. RAID can also be used for SSDs.

However, mirroring two SSDs (as in RAID 1) may not have the desired effect of backing up data, because solid-state drives of the same model and age will likely fail around the same time. If they did, all of the mirrored data would also be lost. SSDs are much less likely to fail than HDDs, and they have much higher read speeds, which helps improve RAID 1’s generally slow read performance.

Jenna Phipps
Jenna Phipps is a contributor for Enterprise Mobile Today, Webopedia.com, and Enterprise Storage Forum. She writes about information technology security, networking, and data storage. Jenna lives in Nashville, TN.

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