SSD Read cache question

[AllenChang]

When I see the domcument

https://docs.netapp.com/us-en/e-series-santricity-116/sm-storage/ssd-cache-and-volumes.html

it say "In SSD Cache, the data is copied from volumes and stored on two internal RAID volumes (one per controller) that are automatically created when you create an SSD Cache."

I think,if there is only one SSD Cache,will system also create two internal RAID volumes?

So I use simulator to make a lab,

I Create two Pool

and create one SSD read cache

then I create three volume,two volume in pool1,and one volume in pool2

and I change the volume's ownership just like below

In this time,I just have only one SSD read cache,

So I enable the SSD read cache in each volume,

and it been enable in simulator,

So my question is,

Why I can enable SSD Cache in each volume with only one SSD Read Cache ?

Is the only one SSD  Read Cache been split two volume and assign to each controller?

[elementx]

> Why I can enable SSD Cache in each volume with only one SSD Read Cache ?

Because SSD cache is not a volume. 

E-Series controller cache is also not a volume, and yet we can enable controller read cache for 100 volumes with just 2 controllers.

Controller cache uses DRAM that belongs to each controller, SSD cache uses SSD media capacity divided between two controllers.

> Is the only one SSD Read Cache been split two volume and assign to each controller?

That's what the documentation quote you mentioned says:

> two internal RAID volumes (one per controller) 

"The internal RAID volumes are used for internal cache processing purposes. These volumes are not accessible or displayed in the user interface." - documentation

[AllenChang]

Hi elemetx

so I just need only one SSD read cache,

than it can serve all volumes in all pool s, and all controller ?

[elementx · Accepted solution]

> so I just need only one SSD read cache

That's what the documentation says:

> Currently, only one SSD Cache is supported per storage array.

Also make sure the rest of restrictions and procedures are acceptable.

https://docs.netapp.com/us-en/e-series-santricity/sm-storage/ssd-cache-restrictions.html

https://docs.netapp.com/us-en/e-series-santricity/sm-storage/create-ssd-cache.html

SSD Cache will help improve read performance only if write workload % is very low (say, below 20%).