Concatenated/JBOD disks in a cache pool?

[augot]

Apologies in advance if I've used the terminology in the subject title incorrectly, but I'm creating this post due to my confusion over exactly what might be the solution to my particular query.

 

I have been using Unraid for several months, and found it excellent for my purposes. My setup currently is 2x8TB WD Red drives as parity, and an array of 3x8TB WD Reds plus 1x3TB Seagate ST3000DM001. (I'm aware of the reliability problems that have been reported with this Seagate model, but it yet to give me any issues.) When I first built my system, I used two old 256GB Sandisk SSDs that I salvaged from a couple of old laptops as a mirrored cache; I abhor e-waste, and try to re-use components wherever possible.

 

My issue is that while my 256GB cache pool has served admirably for hosting my appdata, domains, isos, etc, it is far too small when I am often dealing with files which are 50GB+, and as a result for my actual file shares I have been stuck writing directly to the array each time. This is obviously not ideal in terms of speed nor mitigating drive wear, but it is also an issue for me in terms of power consumption (it's rare that more than two HDDs are able to spin down at any one time) and noise (I have a home studio, and can isolate and deal with irregular spin-up/spin-down noise, particularly when not during working hours; the constant low-level hum of multiple HDDs spinning 24/7 is more problematic.)

 

This week I finally upgraded my cache to a single 1TB Samsung EVO, and it's solved those issues for me. However, I again find myself wanting not to waste my two existing 256GB SSDs; they still work fine, after all. I could add them to the array I guess, but they're both so small that seems somewhat ridiculous (not to mention their speeds would be crippled by having to match HDD speeds).

 

I've been trying to educate myself about different RAID configs, and from my research it seems that there are ways of creating a RAID 1 setup from two "disks" where one of the disks is two or more smaller disks in a RAID0 (or is it more accurately called concatenated? JBOD?) setup. My thinking here is instead of buying another 1TB SSD in the near future and regaining mirroring in the pool for redundancy, I could get a 512GB one at some point and stripe it across with the other two SSDs to create something which is treated by the system as one single 1TB drive (give or take the vagaries of how different manufacturers calculate disk size, of course), and then use that as the other half of my cache pool.

 

Is this something that Unraid would support? (Is it even a good idea?) Apologies if this is documented somewhere already, too, but I haven't been able to find anything on this forum or elsewhere that clarifies this. Any advice would be greatly appreciated.

 

 

[trurl]

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

 

And other topics in the Cache Pool section of the FAQ:

 

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

 

[trurl]

Also the btrfs calculator here:

 

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

[augot]

Wow, somehow I missed that BTRFS actually does this by default in its version of RAID1 with any number of mixed-size drives.

 

Wonderful. Thank you!

[John_M]

54 minutes ago, augot said:

Wow, somehow I missed that BTRFS actually does this by default in its version of RAID1 with any number of mixed-size drives.

Yes, it's pretty impressive, really. My favourite example is a five member RAID10.

[greyday]

Quick question--is it no longer necessary to set up a multi-disk "single" array manually, or will the gui approach still default to R1? The FAQ entry talks about releases 6.2 and 6.3...

[tjb_altf4]

14 minutes ago, greyday said:

Quick question--is it no longer necessary to set up a multi-disk "single" array manually, or will the gui approach still default to R1? The FAQ entry talks about releases 6.2 and 6.3...

BTRFS pools will default to RAID1, after they are created you can rebalance via the GUI to various BTRFS raid levels.
This is the current options in 6.9.2 for a 4 disk BTRFS pool:

 

and for a 2 disk