Storage system requirements for FlashCopy, volume mirroring, and thin-provisionedvolumes

Application performance on a local clustered system can be affected by the use of FlashCopy, volume mirroring, and thin-provisionedvolumes for storage systems.

The FlashCopy, volume mirroring, and thin-provisionedvolume functions can all have a negative impact on system performance. The impact depends on the type of I/O taking place, and is estimated by using a weighting factor from Table 1.

A FlashCopy mapping effectively adds a number of loaded volumes to the storage pool. The effect of mirrored and thin-provisionedvolumes is also estimated in Table 1. The estimates assume that thin-provisionedvolumes are running at approximately 80% capacity of a fully allocated volume, and that mirrored volumes read from one copy and write to all copies.

Table 1. Performance impact estimates for FlashCopy, volume mirroring, and thin-provisionedvolumes
Type of I/O (to volume)	Impact on I/O weighting	FlashCopy weighting	Volume mirroring weighting	Thin-provisioned weighting
None or minimal	Insignificant	0	0	0
Read only	Insignificant	0	0	0.25 * Sv
Sequential read and write	Up to 2 x I/O	2 * F	C−V	0.25 * Sc
Random read and write	Up to 15 x I/O	14 * F	C−V	0.25 * Sc
Random write	Up to 50 x I/O	49 * F	C−V	0.25 * Sc
Notes: In a storage pool with two FlashCopy mappings and random read/write to those volumes, the weighting factor is 14 * 2 = 28. In a storage pool with ten copies, five of which are primary copies of a volume, a weighting factor of 10−5 = 5 applies. If the copies are thin-provisioned, an additional weighting factor of 0.25 * 10 = 2.5 applies. Key: C Number of volume copies in this storage pool V Number of volumes with their primary copy in this storage pool F Number of FlashCopy mappings affecting volumes that have copies in this storage pool Sv Number of thin-provisionedvolume copies in this storage pool that are the primary copy of a volume Sc Number of thin-provisionedvolume copies in this storage pool

To calculate the average I/O rate per volume, use the following equation:

I/O rate = (I/O capacity) / ( V + weighting factor for FlashCopy + 
weighting factor for volume mirroring + weighting factor for thin-provisioned)

For example, consider 20 volumes with an I/O capacity of 5250, a FlashCopy weighting of 28, a mirroring weighting of 5, and a thin-provisioned weighting of 0.25. The I/O rate per volume is 5250 / (20 + 28 + 5 + 2.5) = 94.6. This estimate is an average I/O rate per volume; for example, half of the volumes might be running at 200 I/O operations per second (IOPs), and the other half might be running at 20 IOPs. However, this would not overload the system because the average load is 94.6.

If the average I/O rate to the volumes in the example exceeds 94.6, the system would be overloaded. As approximate guidelines, a heavy I/O rate is 200, a medium I/O rate is 80, and a low I/O rate is 10.

With volume mirroring, a single volume can have multiple copies in different storage pools. The I/O rate for such a volume is the minimum I/O rate that is calculated from each of its storage pool.

If system storage is overloaded, you can migrate some of the volumes to storage pools with available capacity.

Note: Flash drives are exempt from these calculations, except for overall node throughput, which increases substantially for each additional Flash drive in the node.