dm-cache

Tool to create and manage device-mapper cache devices.

DM-cache, or Device Mapper Cache, is a Linux kernel feature designed to enhance storage performance by implementing a block-level cache on a separate cache device.

The goal with dm-cache is to improve random read/write performance of a slow HDD by using a small but fast SSD or NVME device.

The main advantage of dm-cache over lvmcache and bcache is that it is possible to setup on devices that already have a filesystem with data on them. Both LVM and Bcache requires unformatted, empty devices (there are ways to get around, but can be risky).

Requirements

dm-cache requires three devices:

• origin: The slow device.
• cache: A fast SSD or NVME device. Can be of any size.
• meta: A small device that holds dm-cache metadata.

The metadata device size depends on how many cache blocks fit on the cache device. With default setting it should be a least 0.01% of the cache device size. If the cache device is 50GiB, and a cache block size of 128KiB, a metadata device of 5MiB is enough. It is important to have spare space, or dm-cache can become corrupted!

Setup

Using dmcache.sh

Edit dmcache.sh and modify thd following variables:

• dmname: Choose a new name for the assembled dm-cache. It will be exposed as a block device as /dev/mapper/dmname
• origindev: Path to the slow device that shoulf be accelerated with dm-cache. Use a stable device ID, not FS UUID.
• cachedev: The fast cache device, usually an SSD or NVME disk.
• metadev: A small decice to hold cache metadata.
• cachemode: Choose writethrough or writeback cache.

It is important to mount the filesystem on the dm-cache using the /dev/mapper/dmname path and not with the filesystem UUID as is commonly done. This is because the kernel might still see the UUID from the origin device, and this can cause data loss!

Note that the dm-cache mapping is not persistent. After a reboot, the dm-cache must be assembled before the filesystem safely can be mounted.

The writethrough cache mode minimises the risk of problems by only accelerating reads. It makes sure that dirty data is written to the origin device before acknowledging to the application. In theory, it should be possible to use the origin device without dmcache after a reboot or a crash, but it is no guarantee.

Manually stopping dm-cache is done with dmsetup remove <dmname>.

OpenRC

The OpenRC init script can automate setting up and stopping dm-cache during boot.

• Install conf.d/dmcache and `init.d/dmscript
• Modify conf.d/dmcache to suit your setup
• Add a udev rule to block FS UUID device symlinks
• Add dmcache to boot runlevel: rc-update add dmcache boot

Multiple devices

If you have several devices you can simply make a copy of the init.d and conf.d files to a new name. The filenames in init.d and conf.d must be the same.

• cp /etc/conf.d/dmcache /etc/conf.d/dmcache.new
• ln -s /etc/init.d/dmcache /etc/init.d/dmcache.new
• update /etc/conf.d/dmcache.new
• update udev rules
• rc-service dmcache.new start
• rc-update add dmcache.new boot