Make cache drive sense when system is already on SSD?

[ruthan]

Hello,

i have 1 data disc, no cache, no parity config, make sense to use other ssd for cache - will it enhance performance i have here some unused small 64 GB SSD. 

 

I also wonder, is somewere of Unraid disc configuration benchmarks, for example i wont to compare single data disc config vs. 2 data disc + 1 partity disc.

Edited July 11, 2017 by ruthan

[SSD]

If you have an SSD disk1, and no cache, adding an SSD cache is not doing to do anything for you from a performance perspective.

 

There have been some benchmarks done in the past, but they are probably old and pretty useless. There are changes in performance characteristics version over version. Normally small, but occasionally we get a significant change.

 

Parity will definitely slow down writes, but reads are unaffected (full speed). One vs two data disks - doesn't matter. Each disk is independent - there is no striping in unRAID.

 

Write speed impact is dependent on several factors - including your server hardware, speed of data disk, speed of parity, whether using "normal" or "turbo" (a.k.a., "reconstruct") write mode. It also depends on your network speed and other activity on the array. With turbo write, many users are able to saturate a gigabit LAN.

 

There has been some discussion of SSDs as data disks. There is a risk that you could have problems with a parity protected array with them. Parity protects ALL of the disk - at the sector level. SSDs do some behind the scenes things with remapping sectors that can, in theory anyway, impact values in unused space. This would throw off the parity calculation. TBH, I have not seen a lot of people experimenting with SSD arrays, but do believe some people are running them successfully, fully understanding the risk.

 

The goal of parity is to protect you from a failed disk. I highly recommend one if using unRAID as a NAS! Using an array of large spinning disks for media, combined with a relatively small (e.g., 250G - 500G) cache drive for Dockers and VMs, provides extremely fast performance where it matters, and a large amount of inexpensive space for media content. That is the normal use case.