Storage system requirements for FlashCopy, VDisk mirroring, and space-efficient VDisks

Application performance on a local cluster can be affected by the use of FlashCopy®, VDisk mirroring, and space-efficient VDisks for storage systems.

The FlashCopy, VDisk mirroring, and space-efficient VDisk features can all have a negative impact on cluster performance. The impact depends on the type of I/O taking place, and is estimated using a weighting factor from Table 1.

A FlashCopy mapping effectively adds a number of loaded VDisks to the MDisk group. The effect of mirrored and space-efficient VDisks is also estimated in Table 1. The estimates assume that space-efficient VDisks are running at approximately 80% capacity of a fully allocated VDisk, and that mirrored VDisks read from one copy and write to all copies.

Table 1. Performance impact estimates for FlashCopy, VDisk mirroring, and space-efficient VDisks
Type of I/O (to VDisk)	Impact on I/O weighting	FlashCopy weighting	VDisk mirroring weighting	Space-efficient 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 an MDisk group with two FlashCopy mappings and random read/write to those VDisks, the weighting factor is 14 * 2 = 28. In an MDisk group with ten copies, five of which are primary copies of a VDisk, a weighting factor of 10−5 = 5 applies. If the copies are space-efficient, an additional weighting factor of 0.25 * 10 = 2.5 applies. Key: C Number of VDisk Copies in this MDisk Group V Number of VDisks with their primary copy in this MDisk Group F Number of FlashCopy mappings affecting VDisks that have copies in this MDisk Group Sv Number of space-efficient VDisk Copies in this MDisk Group that are the primary copy of a VDisk Sc Number of space-efficient VDisk Copies in this MDisk Group

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

I/O rate = (I/O capacity) / ( V + weighting factor for FlashCopy + 
weighting factor for VDisk mirroring + weighting factor for space-efficient)

For example, consider 20 VDisks with an I/O capacity of 5250, a FlashCopy weighting of 28, a mirroring weighting of 5, and a space-efficient weighting of 0.25. The I/O rate per VDisk is 5250 / (20 + 28 + 5 + 2.5) = 94.6. This estimate is an average I/O rate per VDisk; for example, half of the VDisks could be running at 200 IOPs, and the other half could be running at 20 IOPs. This would not overload the system however, because the average load is 94.6.

If the average I/O rate to the VDisks 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 VDisk mirroring, a single VDisk can have multiple copies in different MDisk groups. The I/O rate for such a VDisk is the minimum I/O rate calculated from each of its MDisk Groups.

If system storage is overloaded, you can migrate some of the VDisks to MDisk groups with available capacity.

Note: Solid-state drives (SSDs) are exempt from these calculations, with the exception of overall node throughput, which increases substantially for each additional SSD in the node.