I am currently testing out unraid on a spare machine to see if it would be a good fit and I think I am mostly ready to pull the trigger but I have a question around the best way to handle a few SSDs I would want to migrate to the build. I can't fully test the scenarios I am looking for since the machine I am testing on only has 3 drives total.

 

I have 4x samsung pro nvme ssd's on a bifurcated PCI express card, I want to take 2 and use them as cache drives and the other two and use them as a separate mirrored volume to keep things I always want to live on SSDs for fast access there and the drives mirrored.

 

At least as I understand it the cache pool is striped and not mirrored so theres a chance of data loss for the persistent ssd data if one of the drives were to fail so I am looking for something a little more stable locally in the machine that can allow me to recover pretty quickly (in addition to my offsite backup which is definitely more of a cold storage type situation.)

 

Am I thinking about this right or is there a better solution that will accomplish what I want? I don't necessarily have a problem with the data permanently in the cache pool and saw the option to store the share only on cache but that doesn't give me the mirroring I am looking for. I don't think unassigned devices would necessarily be the right fit either as that doesn't seem to allow mirroring of multiple drives together. 

By default the cache pool is using the BTRFS form of RAID1 so you do have redundancy.  I think with just 2 drives they end up as mirrors.

 

if you have more than 2 drives then it gets a bit more complicated as BTRFS still ensures everything is on two drives for redundancy, but the split means they are not comply mirrors.

Edited March 25, 20197 yr by itimpi

If you use the unassigned devices on separate hardware mirrored volume or mirrored the other 2 SSD's on the NVMe card via software, that would probably fit the usage needs you described.

See the Cache Drive / Pool section in the FAQ for a lot more info:

 

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?page=2&tab=comments#comment-554741

 

In particular, this one:

 

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

 

Ah perfect, I didn't realize the cache pool was raid 1 by default with multiple drives, I missed that. Some of the documentation references the pool being raid 0 so thats where I got confused. All 4 drives are the same capacity/model/etc. so I think the standard cache pool with all 4 drives would be sufficient for what I am trying to accomplish as long as I set the specific share to be cache only.

 

Thanks for all of the responses.

Edited March 25, 20197 yr by mau471

10 hours ago, mau471 said:

Ah perfect, I didn't realize the cache pool was raid 1 by default with multiple drives, I missed that. Some of the documentation references the pool being raid 0 so thats where I got confused. All 4 drives are the same capacity/model/etc. so I think the standard cache pool with all 4 drives would be sufficient for what I am trying to accomplish as long as I set the specific share to be cache only.

 

Thanks for all of the responses.

Unraid sets up the cache pool as RAID1 by default.    You may have seen some of the posts that talk about how you can manually convert the pool to be RAID0 for those to whom available space is more important than redundancy.