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Figure 7.8 When RAID 0+1 disks fail.
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1. Disk A fails
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2. No other disks are taken offline; data still available 3. Disk C fails
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4. No other disks are taken offline; data sill available E F 5. Disk D fails 6. Then data is lost. Figure 7.9 When RAID 1+0 disks fail.
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RAID-2: Hamming Encoding
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We include this RAID level purely for completeness. RAID-2 uses the same Hamming encoding method for checking the correctness of disk data as is used by Error Correcting Code (ECC) memory. We have been unable to find even one commercial implementation that uses RAID-2. Next.
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RAID-3, -4, and -5: Parity RAID
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RAID levels 3, 4, and 5 are all different styles of parity RAID. They do not require maintaining a complete copy of the original data on a second set of disks, and the associated 100 percent overhead. Instead, each RAID volume requires extra space equivalent to one extra disk. This additional disk s blocks contain calculated parity values, which are generated by taking XORs (eXclusive Ors, a logical Boolean operation) of the contents of the corresponding data blocks on all the other disks in the RAID volume. If any one disk (including the parity disk) in the RAID set is lost, its contents can be calculated based on the contents of all the other disks in the set. If two disks are lost at the same time, or a second disk is lost while the first disk is being rebuilt, all the data on the RAID stripe is lost and must be restored from a backup. The overhead in a parity RAID model is generally only about 20 to 25 percent, depending on the number of disks in the stripe. Parity RAID requires that each member disk in the RAID volume be the same size. Parity RAID introduces significant performance penalties under most conditions. When a write is generated, not only must the write complete, but the updated parity must be recalculated. To recalculate parity, data has to be read from otherwise unaffected disks in the set so that the correct parity value can be written to the parity disk region. This means that in a five-disk RAID-5
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stripe, a single write that fits on one disk causes four reads, five XORs, and two writes. RAID writing, therefore, especially when the writes are small, is generally pretty slow. RAID read performance is basically the same as if no RAID was present, because no parity calculations are required. When a disk fails, its replacement must be rebuilt from the data on the surviving disks, all of which must be read and then processed. Because a rebuild requires reading every block on every disk in the RAID-5 stripe and performing the parity calculations, rebuilds can be extremely time- and CPU-consuming. If Hardware RAID is employed, a lot of CPU cycles will still be required, they will just be on a different CPU, with different performance and cost characteristics from the CPU on the host. The performance of a RAID set is also dependent on the number of disks in the set. Once you exceed six or seven disks in a RAID set, performance will really fall off, because the number of additional reads and calculations that must be performed becomes excessive. The differences between RAID-3, RAID-4, and RAID-5 are primarily in exactly how they implement the parity between the disks.
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RAID-3: Virtual Disk Blocks
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In RAID-3, virtual disk blocks are created and striped across all the disks in the RAID-3 volume. Every disk operation touches all the disks, regardless of the size of the write. The RAID stripe can only process one disk I/O at a time. RAID-3 performance depends on the nature of the writes. If the I/Os are small and random (all over the volume), performance will be poor. If the I/Os are large and sequential, performance will be fast; streaming I/Os work best. Commercial implementations of RAID-3 are generally in hardware rather than software. Storage Technologies (Storage Tek), Medea, and Baydel are three disk array vendors that implement RAID-3 in hardware. For the most part, the vendors use large disk caches to ease any issues with performance.
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