[ale] RAID 1

[Brian Pitts]

Terry Bailey wrote:
> Hi,
> 
> Does RAID 1 slow down performance?

http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1_performance

Additionally, since all the data exist in two or more copies, each with
its own hardware, the read performance can go up roughly as a linear
multiple of the number of copies. That is, a RAID 1 array of two drives
can be reading in two different places at the same time, though not all
implementations of RAID 1 do this.[4] To maximize performance benefits
of RAID 1, independent disk controllers are recommended, one for each
disk. Some refer to this practice as splitting or duplexing. When
reading, both disks can be accessed independently and requested sectors
can be split evenly between the disks. For the usual mirror of two
disks, this would, in theory, double the transfer rate when reading. The
apparent access time of the array would be half that of a single drive.
Unlike RAID 0, this would be for all access patterns, as all the data
are present on all the disks. In reality, the need to move the drive
heads to the next block (to skip unread blocks) can effectively mitigate
speed advantages for sequential access. Read performance can be further
improved by adding drives to the mirror. Many older IDE RAID 1
controllers read only from one disk in the pair, so their read
performance is always that of a single disk. Some older RAID 1
implementations would also read both disks simultaneously and compare
the data to detect errors. The error detection and correction on modern
disks makes this less useful in environments requiring normal
availability. When writing, the array performs like a single disk, as
all mirrors must be written with the data. Note that these performance
scenarios are in the best case with optimal access patterns.