did quick bit of spellchecking. I don't want my drives to strip.

I don't want my drives to strip.

Raid 5 is a combination of striping and mirroring

RAID 5

A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 has achieved popularity due to its low cost of redundancy. Generally, RAID 5 is implemented with hardware support for parity calculations. A minimum of 3 disks is generally required for a complete RAID 5 configuration. A RAID 5 two disk set is possible, but many implementations do not allow for this. In some implementations a degraded disk set can be made (3 disk set of which 2 are online).

In the example above, a read request for block "A1" would be serviced by disk 0. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently by disk 1.

RAID 5 parity handling

A series of blocks (one on each of the disks in an array) is collectively called a "stripe". If another block, or some portion of a block, is written on that same stripe, the parity block (or some portion of the parity block) is recalculated and rewritten. For small writes, this requires reading the old data, writing the new parity, and writing the new data. The disk used for the parity block is staggered from one stripe to the next, hence the term "distributed parity blocks". RAID 5 writes are expensive in terms of disk operations and traffic between the disks and the controller.

The parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error. In this case, the sector in the same relative position within each of the remaining data blocks in the stripe and within the parity block in the stripe are used to reconstruct the errant sector. The CRC error is thus hidden from the main computer. Likewise, should a disk fail in the array, the parity blocks from the surviving disks are combined mathematically with the data blocks from the surviving disks to reconstruct the data on the failed drive "on the fly".

This is sometimes called Interim Data Recovery Mode. The computer knows that a disk drive has failed, but this is only so that the operating system can notify the administrator that a drive needs replacement; applications running on the computer are unaware of the failure. Reading and writing to the drive array continues seamlessly, though with some performance degradation. The difference between RAID 4 and RAID 5 is that in interim data recovery mode, RAID 5 might be slightly faster than RAID 4: When the CRC and parity are in the disk that failed, the calculation does not have to be performed, while with RAID 4, if one of the data disks fails, the calculations have to be performed with each access.

RAID 5 disk failure rate

The maximum number of drives in a RAID 5 redundancy group is theoretically unlimited, but it is common practice to limit the number of drives. The tradeoffs of larger redundancy groups are greater probability of a simultaneous double disk failure, the increased time to rebuild a redundancy group, and the greater probability of encountering an unrecoverable sector during RAID reconstruction. As the number of disks in a RAID 5 group increases, the Mean Time Between Failures (MTBF, the reciprocal of the failure rate) can become lower than that of a single disk. This happens when the likelihood of a second disk failing out of (N-1) dependent disks, within the time it takes to detect, replace and recreate a first failed disk, becomes larger than the likelihood of a single disk failing. RAID 6 is an alternative that provides dual parity protection thus enabling larger numbers of disks per RAID group.

Some RAID vendors will avoid placing disks from the same manufacturing lot in a redundancy group to minimize the odds of simultaneous early life and end of life failures as evidenced by the Bathtub curve.

RAID 5 performance

RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe; this is because parity must be updated on each write, requiring read-modify-write sequences for both the data block and the parity block. More complex implementations often include non-volatile write back cache to reduce the performance impact of incremental parity updates.

The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks. Except for the parity blocks, the distribution of data over the drives follows the same pattern as RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks.

In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe. This potential vulnerability is sometimes known as the "write hole". Battery-backed cache and similar techniques are commonly used to reduce the window of opportunity for this to occur.

RAID 5 usable size

Parity data uses up the capacity of one drive in the array (This can be seen by comparing it with RAID 4: RAID 5 distributes the parity data across the disks, while RAID 4 centralizes it on one disk, but the amount of parity data is the same). In case that the drives vary in capacity, the smallest of them sets the bar. Therefore, the usable capacity of a RAID 5 array is (N-1) \cdot S_{\mathrm{min}}, where N is the total number of drives in the array and Smin is the capacity of the smallest drive in the array.

The number of hard drives that can belong to a single array is theoretically unlimited (although the time required for initial construction of the array as well as that for reconstruction of a failed disk increases with the number of drives in an array).