Pooling cache drives?

[HellDiverUK]

So, I hear there's some way of pooling the cache drives.  As I use my cache drive to run Dockers off as well, I'd like to be able to run two drives in some sort of mirror.  I currently use a 120GB SSD, but that's a little small, so I was thinking of running a pair of 7200rpm 2.5" drives (probably 2x320GB WD Blacks or similar).

 

How do I go about setting this up in 6b12 (or 6b14 when it's released)?

 

I've read lots of threads, but I haven't found a clear answer.  Thanks.

[HellDiverUK]

Wow, really?  No-one?

[meep]

Hi There

 

To use the cache pooling feature in unRAID, your cache drives must be formatted BTRFS.

 

Then, you can assign them to multiple slots under the cache tab.

 

 

There some details in this thread from Limetech which might get you started;

 

http://lime-technology.com/forum/index.php?topic=34794.msg323791;topicseen#msg323791

 

 

There's also this blog entry that explains how it all works, features etc.;

 

https://lime-technology.com/exciting-new-developments-with-unraid-6/

 

 

You might also be interested in the whole roadmap thread on this;

 

http://lime-technology.com/forum/index.php?topic=34391.0

 

 

I played with cache pools when they first appeared in beta7. I don't use them any more but here's a blog entry I wrote at the time;

 

http://mediaserver8.blogspot.ie/2014/08/upgrading-unraid-cache-to-btrfs.html

 

 

Maybe when you get it figured out, you could write a post or wiki entry explaining it for everyone? LimeTech have acknowledged that v6 documentation is lacking but in progress.

 

Good luck!

 

 

 

[Nicktdot]

I'm doing something similar, cache pooling 4x 500GB SSDs in a BTRFS RAID-1.

 

The UI is advanced enough that you can do it all from there, by increasing the allowed # of drives available for cache (by decreasing the qty available for data).

 

By default, as you add cache drives, the BTRFS pooling type will be RAID1 (like), which is smart enough in BTRFS to automatically allocate 1/2 the total space. (unlike true raid-1)

 

it looks something like this:

 

 

 

The drawback from this is that as you pull drive count from the data array to add them to the cache pool, you're decreasing the qty of drives available for data array, which is wrong imo because the cache 'device' is still just 1 drive when it comes to the MD driver... you still mount just one drive (case in point , the mount point from the array pictured above looks like this:)

 

# df /mnt/cache

Filesystem    1K-blocks    Used  Available Use% Mounted on

/dev/sdg1      976771588 91412472 1054255408  8% /mnt/cache

 

 

and the btrfs filesystem looks like this:

# btrfs filesystem show

Label: none  uuid: a97f3ee8-7459-4518-bc4e-8012fae4f360

        Total devices 4 FS bytes used 87.13GiB

        devid    1 size 465.76GiB used 126.03GiB path /dev/sdh1

        devid    2 size 465.76GiB used 126.00GiB path /dev/sde1

        devid    3 size 465.76GiB used 126.03GiB path /dev/sdf1

        devid    4 size 465.76GiB used 126.00GiB path /dev/sdg1

 

I would like for the UI to not take away array drive allocation to allow cache drive pooling.  I'll try to circumvent the UI and see how it works.

 

Some have said that unraid is hard coded to only allow 3 letter drive naming (sda) and thus couldn't support this because drives would go beyond the 26 total physical devices (sdaa, sdab, etc), but that's not true at all, but keeps being repeated.

 

The super.dat config file uses device ID names, not lettering (which is why you can move disks around your array, and they'll start at the correct disk# assignment), and I already run my parity disk as "sdaa" (27th disk) and it runs fine.  more details here http://lime-technology.com/forum/index.php?topic=38189.0

[HellDiverUK]

Thanks for the relies, I'll study them in the morning, it's been a loooooong day.

[HellDiverUK]

OK, finally got to reading the replies.  Thanks Nicktdot and meep, very useful.  I need to decide if I'm going to bother with this or not.  My unRAID plans are a bit in limbo at the moment, I want to change hardware, but I'm not sure what I'm going with.