[6.12] Track btrfs patches

[kakra]

Export patch series: https://github.com/kakra/linux/pull/36.patch

• Allocator hint patches: Allows to prefer SSDs for meta-data allocations while excluding HDDs from meta-data allocation, greatly improves btrfs responsiveness, file system remains compatible with non-patched systems but won't honor allocation preferences then (re-balance needed to fix that after going back to a patched kernel)

To make use of the allocator hints, add these to your kernel. Then run btrfs device usage /path/to/btrfs and take note of which device IDs are SSDs and which are HDDs.

Go to /sys/fs/btrfs/BTRFS-UUID/devinfo and run:

• echo 0 | sudo tee HDD-ID/type to prefer writing data to this device (btrfs will then prefer allocating data chunks from this device before considering other devices) - recommended for HDDs, set by default
• echo 1 | sudo tee SSD-ID/type to prefer writing meta-data to this device (btrfs will then prefer allocating meta-data chunks from this device before considering other devices) - recommended for SSDs
• There's also type 2 and 3 which write meta-data only (2) or data only (3) to the specified device - not recommended, can result in early no-space situations
• Added 2024-06-27: Type 4 can be used to avoid allocating new chunks from a device, useful if you plan on removing the device from the pool in the future: echo 4 | sudo tee LEGACY-ID/type
• NEVER EVER use type 2 or 3 if you only have one type of device
• The default "preferred" heuristics (0 and 1) are good enough because btrfs will always allocate from devices with most space first (respecting the "preferred" type with this patch)
• After changing the values, a one-time meta-data and/or data balance (optionally filtered to the affected device IDs) is needed

Important note: This recommends to use at least two independent SSDs so btrfs meta-data raid1 requirement is still satisfied. You can, however, create two partitions on the same SSD but then it's no longer protected against hardware faults, it's essentially dup-quality meta-data then, not raid1. Before sizing the partitions, look at btrfs device usage to find the amount of meta-data, at least double that size to size your meta-data partitions.

This can be combined with bcache by directly using meta-data partitions as a native SSD partition for btrfs, and only using data partitions routed through bcache. This also takes a lot of meta-data pressure from bcache, making it more efficient and less write-wearing as a result.

Real-world example

In this example, sde is a 1 TB SSD having two meta-data partitions (2x 128 GB) with the remaining space dedicated to a single bcache partition attached to my btrfs pool devices:

# btrfs device usage /
/dev/bcache2, ID: 1
   Device size:             3.63TiB
   Device slack:            3.50KiB
   Data,single:             1.66TiB
   Unallocated:             1.97TiB

/dev/bcache0, ID: 2
   Device size:             3.63TiB
   Device slack:            3.50KiB
   Data,single:             1.66TiB
   Unallocated:             1.97TiB

/dev/bcache1, ID: 3
   Device size:             2.70TiB
   Device slack:            3.50KiB
   Data,single:           752.00GiB
   Unallocated:             1.96TiB

/dev/sde4, ID: 4
   Device size:           128.00GiB
   Device slack:              0.00B
   Metadata,RAID1:         27.00GiB
   System,RAID1:           32.00MiB
   Unallocated:           100.97GiB

/dev/sde5, ID: 5
   Device size:           128.01GiB
   Device slack:              0.00B
   Metadata,RAID1:         27.00GiB
   System,RAID1:           32.00MiB
   Unallocated:           100.98GiB

# bcache show
Name            Type            State                   Bname           AttachToDev
/dev/sdd2       1 (data)        dirty(running)          bcache1         /dev/sde2
/dev/sdb2       1 (data)        dirty(running)          bcache2         /dev/sde2
/dev/sde2       3 (cache)       active                  N/A             N/A
/dev/sdc2       1 (data)        clean(running)          bcache3         /dev/sde2
/dev/sda2       1 (data)        dirty(running)          bcache0         /dev/sde2

A curious reader may find that sde1 and sde3 are missing, which is my EFI boot partition (sde1) and swap space (sde3).