[back to "Optimizing server performance: I/O subsystems"]

Configuring RAID levels for I/O performance (sidebar)
The following sections first show our read/write access test results, and then describe the different hardware RAID levels.

Read/write access test results
To verify the appropriate RAID level to use for the various workloads, we tested the read and write access rates on our Sun Solaris system. Our results show that the write percentage is high for the workloads, so we recommend using the RAID0+1 or RAID1 Enhanced levels, not RAID5. (Note: The specific read/write access numbers may vary on other platforms.)

Workload	Writes (%)	Reads (%)	Writes Transfer Size (KB)	Reads Transfer Size (KB)
R5Mail	60	40	14KB	22KB
Webmail	90	10	8KB	26KB
R5IMAP	90	10	12KB	16KB
SMTP/POP3	70	30	12KB	38KB

About the hardware RAID levels
To configure your hardware RAID levels for optimum I/O performance, you need to understand the five basic RAID levels: RAID0, RAID1, RAID0+1, RAID1 Enhanced, and RAID5.

• RAID0 stripes data across multiple disk drives -- stripe1 is on drive1, stripe2 is on drive2, and so on -- by using the stripe or chunk size. There is no redundancy of data, and there is no penalty for writes.
• RAID1 is a mirrored set of two physical disk drives. Applications can read from either drive. However, they must write to both drives, resulting in extra I/O for writes.
• RAID0+1 is mirroring, plus striping -- a combination of RAID0 and RAID1. Hardware RAID controllers mirror pairs of disk drives, and then stripe the pairs. For example, for six members in the LUN, drives 1 and 2 are mirrored (m1); drives 3 and 4 are mirrored (m2); and drives 5 and 6 are mirrored (m3). Then, m1, m2, and m3 are striped. Usable space is 50 percent of the total space. You need an even number of disk drives for RAID0+1. Applications can read from the data or the mirrored copy. They must write both to the data and copy, resulting in an extra write.
• RAID1 Enhanced is striping, then mirroring the stripe. The first stripe is the data stripe; the second stripe is the mirror (copy) of the first data stripe, but shifted by one drive. Usable space is 50 percent of the total space. You can use an odd number of disk drives for RAID1 Enhanced. Applications can read from the data or the copy. They must write both to the data and copy, resulting in an extra write. (Note: A hardware vendor usually supports RAID0+1 or RAID1 Enhanced. The performance of RAID0+1 and RAID1 Enhanced should be the same.)
• RAID5 is striping data and parity across all members of the RAID set. It requires the extra storage of one disk drive for parity. For each write operation, four disk I/Os are necessary to:
  1. Read the data block.
  2. Read the parity block (and calculate the new parity block, which is not a disk I/O)
  3. Write the updated data block.
  4. Write the updated parity block.
  
  Therefore, RAID5 requires three additional disk I/Os.