HTGWA: Use bcache for SSD caching on a Raspberry Pi

This is a simple guide, part of a series I'll call 'How-To Guide Without Ads'. In it, I'm going to document how I set up bcache on a Raspberry Pi, so I could use an SSD as a cache in front of a RAID array.

Getting bcache

bcache is sometimes used on Linux devices to allow a more efficient SSD cache to run in front of a single or multiple slower hard drives—typically in a storage array.

In my case, I have three SATA hard drives: /dev/sda, /dev/sdb, and /dev/sdc. And I have one NVMe SSD: /dev/nvme0n1.

I created a RAID5 array with mdadm for the three hard drives, and had the raid device /dev/md0.

I then installed bcache-tools:

$ sudo apt-get install bcache-tools

And used make-bcache to create the backing and cache devices:

$ sudo make-bcache -B /dev/md0
UUID:           eb360a2d-4c62-451d-8549-a68621c633e5
Set UUID:       c8b5c63c-0a44-49f3-bb65-cd4df9b751a0
version:        1
block_size:     1
data_offset:        16

$ sudo make-bcache -C /dev/nvme0n1
UUID:           15bf54e9-be21-4478-b676-a08dad937963
Set UUID:       dea419ba-d795-4566-b01f-bb57fa96eb21
version:        0
nbuckets:       15261770
block_size:     1
bucket_size:        1024
nr_in_set:      1
nr_this_dev:        0
first_bucket:       1

Then I tried to look in /sys/block/md0/bcache/ so I could attach the cache to the backing device, but I realized bcache isn't loaded into the default Raspberry Pi OS kernel... so I'll have to compile that in.

Getting bcache on Raspberry Pi OS

I cross-compiled the Raspberry Pi Linux kernel, and when I did it, during the menuconfig portion, I selected the following option:

> Device Drivers
  > Multiple devices driver support (RAID and LVM)
    > Block device as cache (BCACHE)

I recompiled the kernel and copied my updated kernel to the Pi, then rebooted.

At this point, I could see the bcache0 device was working:

pi@omv:~ $ lsblk
sda           8:0    0  3.6T  0 disk  
└─md0         9:0    0  7.3T  0 raid5 
  └─bcache0 254:0    0  7.3T  0 disk  /mnt
sdb           8:16   0  3.6T  0 disk  
└─md0         9:0    0  7.3T  0 raid5 
  └─bcache0 254:0    0  7.3T  0 disk  /mnt
sdc           8:32   0  3.6T  0 disk  
└─md0         9:0    0  7.3T  0 raid5 
  └─bcache0 254:0    0  7.3T  0 disk  /mnt
mmcblk0     179:0    0 14.8G  0 disk  
├─mmcblk0p1 179:1    0  256M  0 part  /boot
└─mmcblk0p2 179:2    0 14.6G  0 part  /
nvme0n1     259:0    0  7.3T  0 disk  

But if I checked on the status of the cache, it said there was no cache:

pi@omv:~ $ cat /sys/block/bcache0/bcache/state
no cache

Attaching the SSD cache to the backing device

Finally, it's time to attach the SSD cache to the backing device:

$ sudo su  # switch to the root user
# cd /sys/block/md0/bcache/
# echo dea419ba-d795-4566-b01f-bb57fa96eb21 > attach
# cat state 

The UUID in the echo command above comes from the 'Set UUID' output from the make-bcache -C command earlier.

Creating a filesystem and mounting

To actually use the device, I formatted it and mounted it to /mnt:

$ sudo mkfs.ext4 -E lazy_itable_init=0,lazy_journal_init=0 /dev/bcache0
$ sudo mount /dev/bcache0 /mnt

To avoid the initialization when making the filesystem, you can omit the -E option entirely. But for RAID arrays I typically let it go full blast on first initialization, because I don't like relying on ext4lazyinit on a RAID array—it can take days at its reduced rate, and affect RAID performance that whole time!

Getting stats

You can check the stats from bcache with:

$ tail /sys/block/bcache0/bcache/stats_total/*
==> /sys/block/bcache0/bcache/stats_total/bypassed <==

==> /sys/block/bcache0/bcache/stats_total/cache_bypass_hits <==

==> /sys/block/bcache0/bcache/stats_total/cache_bypass_misses <==

==> /sys/block/bcache0/bcache/stats_total/cache_hit_ratio <==

==> /sys/block/bcache0/bcache/stats_total/cache_hits <==

==> /sys/block/bcache0/bcache/stats_total/cache_miss_collisions <==

==> /sys/block/bcache0/bcache/stats_total/cache_misses <==

==> /sys/block/bcache0/bcache/stats_total/cache_readaheads <==

Switching the caching mode

There are multiple caching modes, including writeback, writethrough, writearound, and none. The most performant (but most dangerous, especially if you're using a single SSD and not a set of SSDs in RAID 1 for safety) is writeback, which caches reads, and writes data to the SSD first (considering a write 'complete' once written to the SSD), then asynchronously copies that data to the backing device.

Check the current caching mode with:

$ sudo cat /sys/block/bcache0/bcache/cache_mode
[writethrough] writeback writearound none

To change it, for example, to writeback:

$ sudo su - -c 'echo writeback > /sys/block/bcache0/bcache/cache_mode'

Dropping the cache

If you want to pop the SSD off of the backing device, and use it again for other purposes, you have to first de-register it (otherwise you'll get errors like probing initialization failed: Device or resource busy):

$ sudo su
# cd /sys/block/md0/bcache
# echo 1 > detach  # Prints a 'cached_dev_detach_finish' message in `dmesg` log
# cd /sys/fs/bcache/dea419ba-d795-4566-b01f-bb57fa96eb21
# echo 1 > stop  # Prints a 'cache_set_free ' message in `dmesg` log

Then if you want to use the device for something else, wipe it with wipefs:

# wipefs -a /dev/nvme0n1

See the kernel documentation for bcache for more detail and usage examples.