2 SSD for Cache

[maxermaxer]

I recently purchased QNAP TS-653PRO and 32TB 5400rpm harddisk. I want it to be a backup-machine but I still need a bit of speed. I read the guide in QNAP. (QNAP Storage Performance Best Practice :: Storage ::NAS :: QNAP). What is says is that I can use two SSD as cache. I am not so clear why 2 is better. I have two DC grade SSDs to use as cache if needed. But just want to know why two SSD as cache is better than 1. Thanks!

 

[i386]

My guess without reading the link is redudancy. In case of a failed ssd the data can be written from the second ssd to the array without data loss.

 

[William]

I believe they use one for reads and one for writes.

 

[voodooFX]

it is requested for safety reason.
when you write something, it's first written in the SSD pool (cache) and when it finishes your application will get a "OK done" message from the NAS but your data will still be on the cache.
at this point data is moved (flushed) to the disk pool but, if during this process your SSD fails, data will be lost o corrupt.
to avoid this RAID1 (or 10) is requested for SSD write caches.