I have a BTRFS filesystem with a HDDs of various sizes. I thought adding an SSD might allow BTRFS to use it as a caching layer or just another much-faster drive (the docs are a little sparse on this: https://btrfs.wiki.kernel.org/index.php/FAQ#Is_Btrfs_optimized_for_SSD.3F ). However when I run btrfs fi show /data it shows the SSD as unused:
Total devices 8 FS bytes used 4.29TiB // SSD moved to the top of the list for easier reading devid 9 size 223.57GiB used 0.00B path /dev/sda devid 1 size 2.73TiB used 1.28TiB path /dev/sdb1 devid 5 size 0.00B used 356.00GiB path /dev/sdi1 devid 6 size 1.82TiB used 379.00GiB path /dev/sde1 devid 7 size 4.55TiB used 3.09TiB path /dev/sdh devid 8 size 3.64TiB used 2.19TiB path /dev/sdc devid 10 size 2.73TiB used 1.28TiB path /dev/sdf devid 11 size 1.82TiB used 24.00GiB path /dev/sdg
Also if I run btrfs device usage /data it also reports the SSD as unused:
/dev/sda, ID: 9 Device size: 223.57GiB Unallocated: 223.57GiB
So I thought I'd check my fstab mount options:
# Mount BTRFS array UUID=587d228b-5dc3-44e6-b9d9-83df5682d50d /data btrfs defaults,noatime,space_cache,discard 0 0
As far as I can tell these are all pretty standard.
Is this normal, how can I tell if BTRFS is actually using the drive at all? (I may be totally mistaken and BTRFS might not even support this use case at all)