I've had an UnRAID server for years but only recently decided to add cache drives/pool as I was seeing it come up more an more on UnRAID tutorials as a good way to reduce disk spinups.

I have added two SSD drives (2TB and 1TB) to UnRAID as a RAID1 btrfs pool, giving me 1.5TB of usable space. These are repurposed drives that were no longer in use but have relatively low power on hours.

I read of the UnRAID guide here [ https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/cache-pools/ ] but that provided no info about implimentation.

What my expectations/understandings of a cache drive/pool would be that any data copied to the array (including movies for PLEX) will go to the cache drives and then at some speicifed point in time, or as cache pool reaches X amountof storge remaining, that it copies these files to the actual array. Is this a correct assumption or do I misunderstand what a cache drive/pool is supposed to do?

Am I also to understand that I should copy all of my exisitng application data from my array to the cache pool? Will doing this break the Dockers that I have in place, or will nothing really change as they will still point to the same shares/data on the array?

Happy to follow a proper guide, video or forum post if anyone has one and doesn't feel like repaeting what has likely been asked many times before.

Thank you for any assistance with this.

25 minutes ago, 3560freak said:

two SSD drives (2TB and 1TB) to UnRAID as a RAID1 btrfs pool, giving me 1.5TB of usable space

btrfs raid1 is a mirror and only gives total capacity equal to the smaller of the 2 disks.

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#findComment-480420

27 minutes ago, 3560freak said:

any data copied to the array (including movies for PLEX) will go to the cache drives and then at some speicifed point in time, or as cache pool reaches X amountof storge remaining, that it copies these files to the actual array.

Mover runs on schedule and not based on how much cache has remaining. Be sure to read the Minimum free space section at that link you gave.

Each user share has settings that control whether and how it uses cache. Default is to not cache.

The rest of the docs about cache is in the User Shares section here:

https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/shares/#user-shares

38 minutes ago, 3560freak said:

copy all of my exisitng application data from my array to the cache pool? Will doing this break the Dockers that I have in place, or will nothing really change as they will still point to the same shares/data on the array?

In addition to caching user share writes, cache or other pool is useful to get better performance from Docker/VM and to allow array disks to spin down since Docker/VM will keep some files open all the time. The relevant shares are appdata, domains, and system. Nothing can move open files so you have to disable Docker and VM Manager in Settings to work with those shares.

If you want more specific advice based on your configuration...

Attach Diagnostics ZIP to your NEXT post in this thread.

2 minutes ago, trurl said:

Mover runs on schedule and not based on how much cache has remaining. ...

Each user share has settings that control whether and how it uses cache

Mover is intended for idle time. It is impossible to move from cache to array as fast as you can write to cache. If you try to write to cache and move from cache at the same time, it just makes things worse. If you need to write more at one time than cache can hold, don't cache.

9 minutes ago, trurl said:

Mover runs on schedule

You can also run Mover yourself with the MOVE button in Array Operation.

11 hours ago, trurl said:

btrfs raid1 is a mirror and only gives total capacity equal to the smaller of the 2 disks.

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#findComment-480420

I wonder why UnRAID shows two different sizes in the pool? Total size shows 1.5TB but Free Space shows only 1TB.

11 hours ago, trurl said:

Mover runs on schedule and not based on how much cache has remaining. Be sure to read the Minimum free space section at that link you gave.

Each user share has settings that control whether and how it uses cache. Default is to not cache.

The rest of the docs about cache is in the User Shares section here:

https://docs.unraid.net/unraid-os/using-unraid-to/manage-storage/shares/#user-shares

In addition to caching user share writes, cache or other pool is useful to get better performance from Docker/VM and to allow array disks to spin down since Docker/VM will keep some files open all the time. The relevant shares are appdata, domains, and system. Nothing can move open files so you have to disable Docker and VM Manager in Settings to work with those shares.

If you want more specific advice based on your configuration...

Attach Diagnostics ZIP to your NEXT post in this thread.

I'll take a closer look into the User Shares section that you referenced.

Thank you for the replies.

8 minutes ago, 3560freak said:

I wonder why UnRAID shows two different sizes in the pool? Total size shows 1.5TB but Free Space shows only 1TB.

This is normal with different-sized devices; used and free space are the ones you need to look at.