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
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
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
clean
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 <==
563.1M
==> /sys/block/bcache0/bcache/stats_total/cache_bypass_hits <==
0
==> /sys/block/bcache0/bcache/stats_total/cache_bypass_misses <==
0
==> /sys/block/bcache0/bcache/stats_total/cache_hit_ratio <==
13
==> /sys/block/bcache0/bcache/stats_total/cache_hits <==
132
==> /sys/block/bcache0/bcache/stats_total/cache_miss_collisions <==
0
==> /sys/block/bcache0/bcache/stats_total/cache_misses <==
849
==> /sys/block/bcache0/bcache/stats_total/cache_readaheads <==
0
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.
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