Storage system requirements for FlashCopy, volume mirroring, and thin-provisioned volumes
Application performance on a local clustered system can be affected by the use of FlashCopy®, volume mirroring, and thin-provisioned volumes for storage systems.
The FlashCopy, volume mirroring, and thin-provisioned
volume 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-provisioned volumes is also
estimated in Table 1. The estimates assume
that thin-provisioned volumes are running at approximately 80% capacity of a
standard-provisioned
volume, and that mirrored volumes read from one copy and write to all copies.
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:
Key:
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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 rate 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.