مفاهیم FAST Cache و FAST VP در استوریج های EMC
It might be a little confusing at the beginning to understand the difference between FAST Cache and FAST VP. Both are created from FLASH drives, and both have very similar, performance wise, advantages. But those are different technologies. First of all, FAST Cache you set up on Storage System level, so everything provisioned on the Storage Array can take an advantage, both RAID LUNs and pool LUNs. FAST VP is created in a Storage Pool, and can be used only for LUNs within that Storage Pool.
Take a look at comparison created by EMC to see all the main differences.
|FAST CACHE||FAST VP|
|Allows Flash drives to be used to extend the existing caching capacity of the whole storage system||Allows a single LUN to leverage the advantages of multiple drive types through the use of storage pools.|
|Granularity is 64 KB||Granularity is 1 GB|
|Data that is accessed frequently iscopied from HDDs to Flash drives||Data is moved between different storage tiers based on statistics collected over a period of time|
|Use when workload changes are unpredictable and very dynamic, and require a quick response time||Use when workload pattern changes are predictable and relatively low.|
|Constantly promotes frequently accessed HDD data to FAST Cache. There are no relocation cycles.||Data movement occurs in scheduled or manually invoked relocation windows|
|Calculation to decide which data needs to be promoted to FAST Cache is performed continuously.||Calculation to decide which portion of data need to be moved is performed once every hour.|
Of course you can use both at the same time: FAST Cache and FAST VP to yield high performance from the storage system. For example, you can use Flash drives to create FAST Cache, and use FAST VP for storage pools consisting of SAS and NL-SAS drives. Or even devide your Flash drives into two groups: one for FAST Cache usage, and second to use it in ‘extreme performance’ tier for your important storage pool.
If you use both FAST VP and FAST Cache, if FAST VP moves a chunk of data to Flash drives, FAST Cache will not promote that chunk of data into FAST Cache.