HP Smart Array Controllers and basic RAID performance factors

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With RAID 5, each high-level write operation to the logical drive takes several lower level operations. As Table 3 shows,
each RAID 5 write takes four low level drive operations and a parity calculation. In the worst case, RAID 5 random write
performance could be only one-quarter that of a single RAID 0 drive.
Table 3. Breakdown of a RAID 5 high-level write operation
Low-level operation Purpose
Read data drive Retrieve current data
Read parity drive Retrieve current parity information
Compute new parity Based on current data and parity plus new data
Write data drive Write new data values to the data drive
Write parity drive Write new parity values to parity drive
Write operations for RAID 5 and RAID 6 levels
RAID 6, also known as Advanced Data Guarding (ADG), calculates two independent forms of parity. This creates two
parity strips in each data stripe across (see Figure 10). With RAID 6, an array of N drives can store N – 2 drives of data.
Any two drives can fail and the data in the array can still be mathematically reconstructed.
Figure10. Configuration of a RAID 6 drive array
With RAID 6, the write performance penalty is greater than with RAID 5. Each high-level write operation to the logical
drive requires executing six low-level drive read/write operations and two separate parity calculations. Random write
performance for a RAID 6 logical drive could be one-sixth of an equivalent RAID 0 logical drive.
Write operations for Advance Data Mirroring; RAID 1ADM and RAID 1+0ADM
RAID 1 ADM is similar to RAID 1, in that it uses mirrored copies of each drive for data protection. ADM carries this
protection one level further than RAID 1 by using two mirror copies of each drive. This means that for every “write” of a
block of data to a logical drive, the Smart Array controller must execute three low-level writes, one to each of the
mirrored drives. In a simple non-cached example, this would mean that in the worst-case scenario, write performance
will be one third that of writing to a non-arrayed physical drive. With RAID 1 ADM there is no striping. This reduces the
array controller’s ability to execute writes in parallel across multiple physical drives, which results in lower performance
than RAID 0; or similar to the performance of a single drive.
RAID 1+0 ADM mirrors data; however, it also striped the data across the mirrored drive sets. This distributes the data
evenly across the drives and provides better write performance. RAID 1+0 ADM requires executing three low-level writes
for each high-level write to the logical drive.