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HP Smart Array Controllers and basic RAID performance factors

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RAID performance
Most RAID levels improve read performance by distributing, or “striping,” data across a set of physical drives that have
been configured as a single logical drive. Striping places a pre-determined amount of data onto a different physical drive
in the array on a rotating basis. A “strip” is the amount of data written to each drive. A “stripe” is one complete row of
data strips across all of the drives in an array.
The strip size for an array is configurable, and can be set from 16 KiB up to 1024 KiB. In general, using a larger strip size
delivers higher performance for a drive array. The Array Configuration Utility (ACU) determines the largest strip size that
can be set for a given logical array based on the RAID level of the array and the number of physical drives that it
contains.
Read performance
Drive arrays increase the read performance by distributing (striping) data across multiple drives. Because the data is
distributed, the Smart Array controller can execute read operations in parallel. Neither the Smart Array processor nor
the cache size affects the read performance greatly. The drive performance has the greatest impact on the read
performance for Smart Array drive arrays.
Random read performance
Two key factors influence drive array read performance, particularly random read performance:
• Data striping
• The number of drives in the array
Data striping distributes data evenly across all the drives in an array. Data striping improves performance because the
Smart Array controller executes the read requests in parallel across drives. RAID 0, RAID 5, and RAID 6 have similar
overall read performance because they all use data striping.
The number of small (4 KiB to 8 KiB) random IOPS affects the random read performance.
Random read performance for RAID 0, RAID 10, RAID 5, and RAID 6 scales almost directly with drive count. With all other
factors being equal, a 12-drive array can deliver approximately four times random IOPS as an array with three drives.
RAID 1+0 (RAID 10) uses striping and mirroring. Its performance scales linearly with the drive count. Because it is
mirrored as well as striped, RAID 1+0 requires two physical drives to achieve the same net increase in data storage
capacity as a single additional drive does for RAID 0, 5, or 6.