How does one calculate how much flash storage is required??

[Tom Lyczko]

We need to determine whether and if to augment our two-node HP VSA 12.6 cluster with flash/ssd drives to use adaptive optimization.

How do we calculate how much flash we need?? I've seen the figure 10-20% -- but 10-20% of what???

The two servers are both HP DL380 Gen8 2U LFF drive servers which presently contain 15k 600GB SAS drives.

Thank you, Tom

[Stor_Mort]

Hi Tom,

Provision 10-20% of the total capacity in Tier 0. 

For example, if you have eight 600GB SAS raid 5 drives providing 4200GB of raw storage in Tier 1, 10-20% would be 420GB to 840GB. It would be reasonable to add four raid 1 400GB SSD for 800 GB of raw storage. That would give you 800/5000 or 20% of total storage in Tier 0 SSD,

The best ratio will depend on your application behaviour. If most of the frequently accessed storage block addresses total less than 400GB, you might get adequate results with 400GB of SSD storage. If the storage is accessed over a wide range of block addresses, more SSD would make sense.