Back up Your Backups - RAID, Shmade
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I used to think that so long as I had a RAID configuration, my data would be safe and I could sleep at night. After all that’s what the hosting companies tout the most for data backup – making sure all of your hard drives are in a RAID configuration. Raid is defined by Webopedia below:
“Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.”
Now there are a number of different types of RAID with which you should make sure you are familiar:
- Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails, then all data in the array is lost.
- Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
- Level 2 -- Error-Correcting Coding: Not a typical implementation, and rarely used, Level 2 stripes data at the bit level rather than the block level.
- Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
- Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
- Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
- Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.
- Level 0+1 – A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
- Level 10 – A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
- Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
- RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.
The types of RAID we want to concern ourselves with the most for data back up purposes are RAID 1 and RAID 5 configurations. RAID alone should not make you all warm and fuzzy. What a RAID configuration will help you with the most is if one drive fails. Then you should have a mirrored drive which you can then rebuild the data from with no data lost.
That is in theory. Many times a RAID controller will become bad, possibly from an electric short or spike, dust, an ant (did I say ant, yes I did!) or some other malfunction which can cause damage to all drives connected to it simultaneously. Sound far fetched? It isn’t – this has happened to us at least on three different occasions!
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