Question about adding a new SSD Cache Disk...

[questionbot]

Hi there... when I set up my server I put in a 250GB SSD... it seems to work fine but I find that the disk maxes out a bit quickly. So I was thinking of adding another SSD. The thing is my understanding is that if I add another disk it automatically tries to make it a parity disk for the cache. I was thinking I would like to add a 1TB SSD or at the least a 512GB SSD....

 

TL;DR : I would like to add a 2nd SSD to my unraid to expand the cache size... can I just put a new disk in the system.. or are there other considerations I need to understand.

 

Thanks.

[Squid]

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?tab=comments#comment-480421

 

[questionbot]

I'm sorry the FAQ is to technical for me, I do not really understand what that post is saying at all.

[JorgeB]

9 hours ago, questionbot said:

The thing is my understanding is that if I add another disk it automatically tries to make it a parity disk for the cache.

By default it creates a raid1 pool, if you use two different size devices pool will have the capacity of the smaller one, you can change the profile to a different one to use the full capacity (available options are in the linked FAQ entry), at the expense of redundancy.

[ChatNoir]

 

51 minutes ago, questionbot said:

I'm sorry the FAQ is to technical for me, I do not really understand what that post is saying at all.

As for the complexity of the FAQ entry, you should be able to ignore everything after the image if you are on 6.8.3 or above.

It is not really difficult:

• Stop the array
• add the second disk to the pool
• start the array
• Unraid will automatically balance for RAID1, wait for the balance to complete
• balance to RAID0 if you don't mind the lack of redundancy. It is easy, just select RAID0 from the drop down list and hit Balance.
• wait for the balance to complete
• You have a 2 devices RAID0 cache.

[questionbot]

So I really need to add 2 more ssds... say a 512x2... that would give me 512+250+Redundancy?

[JorgeB]

This will tell what you get for any possible device combination:

https://carfax.org.uk/btrfs-usage/

[questionbot]

On 11/3/2020 at 7:08 PM, JorgeB said:

This will tell what you get for any possible device combination:

https://carfax.org.uk/btrfs-usage/

I do not know what all that stuff means... "preset raid levels" and stuff... or what data stripes or parity stripes are.

 

If I remove the 25GB and add 2x1TB... that will work right? As long as I have done "move" before hand?

Edited December 20, 2020 by questionbot

[GPT347]

RAID0 means that if either drive fails, you have lost your cache.  It's best to do RAID1 which mirrors the data on both disks, meaning you could lose either and still have a second copy.  In order to mirror, you should have disks of equal size.  If you have a 256GB disk and a 1TB disk, you can only mirror 256GBs, meaning you have wasted the other 3/4 of the 1TB disk.  If you have 2x1TB mirrored (RAID1), you will have full redundancy with no wasted space. HTH