SSD's as array, mech. disk parity... Thoughts, considerations & abuse

[Nasha]

Hi All, 

 

Haven't had a NAS at home for ages now and I finally sold off all my existing server HW, so I have a bit of a budget to start planning things again. I've ended up with a bunch (6+) of 120GB SSD's and I wanted something small this time around, so an SSD based array seemed a logical choice, especially given I don't have massive data requirements (no 4k RAW family home video libraries here). Of course id look at getting some larger drives as well, which brought me to the "do I need to waste $$'s on an SSD parity drive?" question to myself. 

 

After some pondering, I assumed this would be not only frowned upon, but may actually be a bad idea... Is it really a genuinely bad idea, or just not the best choice. Money is the driving factor, not alot to spare so buying a pair of ~500GB SSD's, and using a shucked 2TB portable HDD for parity would ideally be disks done for me. 

 

So I'd like to know what nasties am I opening myself up to going down this route? 

 

I appreciate any and all feedback, and thanks for reading! 

[MAM59]

overall I would consider this a very very bad idea.

SSDs in an array, not good with UNRAID.

The problem is, the TRIM command is not applied to drives in the array, so in the long run, your array will start to crawl like a snake at the north pole...

 

The other bad idea is the parity drive. you need to "waste $$ for an SSD parity drive" simply because slow drives will slow down writing speed. and if the slowest drive is the parity drive, it will slow down ALL others.

So, if you do not use an SSD or faster drive as parity, you wont ever see the SSD speed of your array.

 

And, of course, the TRIM problem is the same for the parity drive. Its even worse here because a parity drive is always used up to the max. If you do this with a "same size" SSD, it will instantly slow down and never speed up again.

You might overcome this with a bigger parity SSD, blocks that are never used can be taken like "overprovisioning" (but even this will end some day).

 

You COULD use those SSDs as "pool disks", without parity (but maybe mirrored or RAID-ed), but not inside an array (yet, rumors were heard that it will be available in future Versions of UNRAID...)

 

[Nasha]

Ok... I figured that speed thing might be a potential issue... But no Trim I didn't see coming... I'll admit I have very little idea what this means, but I know it's not good when it's missing. I was only going the SSD route because I have all these drives here, but maybe I sell them off and buy some ex-server 3/6/900GB 10k SAS drives which can be had pretty cheap. I just need to make sure their thickness is going to fit my case and mounting bracket. I think that may well be an issue, which may mean back to the drawing board. Ho Hum! 

 

Thanks for the info

[JonathanM]

3 hours ago, Nasha said:

no Trim I didn't see coming... I'll admit I have very little idea what this means, but I know it's not good when it's missing.

High end and newer SSD's may not need trimming, as they have very good internal controller management. However, your "bunch" of small old SSD's probably would need trimming to stay at their best.