Visual Raid Representation
Write Operations
In both mirrored and parity RAID configurations, every write operation translates into more I/O overhead for the disks, which is referred to as a write penalty. In a RAID 1 implementation, every write operation must be performed on two disks configured as a mirrored pair, whereas in a RAID 5 implementation, a write operation may manifest as four I/O operations. When performing I/Os to a disk configured with RAID 5, the controller has to read, recalculate, and write a parity segment for every data write operation.
Figure 3.11 illustrates a single write operation on RAID 5 that contains a group of five disks.
Figure 3.11 illustrates a single write operation on RAID 5 that contains a group of five disks.
Information Storage and Management: Storing, Managing, and Protecting Digital Information in Classic, Virtualized, and Cloud Environments, Second Edition
3.5 RAID Impact on Disk Performance
3.5 RAID Impact on Disk Performance
When choosing a RAID type, it is imperative to consider its impact on disk performance and application IOPS.
In both mirrored and parity RAID configurations, every write operation translates into more I/O overhead for the disks, which is referred to as a write penalty. In a RAID 1 implementation, every write operation must be performed on two disks configured as a mirrored pair, whereas in a RAID 5 implementation, a write operation may manifest as four I/O operations. When performing I/Os to a disk configured with RAID 5, the controller has to read, recalculate, and write a parity segment for every data write operation.
Figure 3.11 illustrates a single write operation on RAID 5 that contains a group of five disks.
Figure 3.11 Write penalty in RAID 5
The parity (P) at the controller is calculated as follows:
Whenever the controller performs a write I/O, parity must be computed by reading the old parity (Ep old) and the old data (E4 old) from the disk, which means two read I/Os. Then, the new parity (Ep new) is computed as follows:
Write Penalty
After computing the new parity, the controller completes the write I/O by writing the new data and the new parity onto the disks, amounting to two write I/Os. Therefore, the controller performs two disk reads and two disk writes for every write operation, and the write penalty is 4.
In RAID 6, which maintains dual parity, a disk write requires three read operations: two parity and one data. After calculating both new parities, the controller performs three write operations: two parity and an I/O. Therefore, in a RAID 6 implementation, the controller performs six I/O operations for each write I/O, and the write penalty is 6.
3.5.1 Application IOPS and RAID Configurations
When deciding the number of disks required for an application, it is important to consider the impact of RAID based on IOPS generated by the application. The total disk load should be computed by considering the type of RAID configuration and the ratio of read compared to write from the host.
The following example illustrates the method to compute the disk load in different types of RAID.
Consider an application that generates 5,200 IOPS, with 60 percent of them being reads.
The disk load in RAID 5 is calculated as follows:
RAID 5 disk load (reads + writes) = 0.6 x 5,200 + 4 x (0.4 x 5,200) [because the write penalty for RAID 5 is 4]
= 3,120 + 4 x 2,080
= 3,120 + 8,320
= 11,440 IOPS
The disk load in RAID 1 is calculated as follows:
RAID 1 disk load = 0.6 x 5,200 + 2 x (0.4 x 5,200) [because every write manifests as two writes to the disks]
= 3,120 + 2 x 2,080
= 3,120 + 4,160
= 7,280 IOPS
The computed disk load determines the number of disks required for the application. If in this example a disk drive with a specification of a maximum 180 IOPS needs to be used, the number of disks required to meet the workload for the RAID configuration would be as follows:
RAID 5: 11,440/180 = 64 disks
RAID 1: 7,280/180 = 42 disks (approximated to the nearest even number)
Disk IOPs and Penalties
Here are the write iops penalty per raid group:
raid 0 – no penalty (however no parity drives for recovery either though)
raid 1 or raid 10 – 2x write penalty (multiply number of writes required by 2),
raid 5 – 4x write penalty,
raid 6 – 5x write penalty.
some disk averages:
(from a Dell Consultant) -
Low End was for a MD3000i
High End was for a equallogic
7200RPM 70 iops - 85 iops
10K RPM 110 iops - 130 iops
15K RPM 150 iops - 180 iops
Parity-based RAID systems also introduce other additional processing that result from the need to calculate parity information. The more parity protection you add to a system, the more processing overhead you incur. As you might expect, the overall imposed penalty is very dependent on the balance between read and write workloads.
