SSD cache
Summary
Using recent kernel (3.9 and later) features for (fast) SSD caching of (slow) ordinary hard disks. Currently two kinds of SSD caching are available in the Linux kernel: bcache and dm-cache. In Fedora 20 basic bcache support is provided by the "bcache-tools" package. dm-cache support was postponed until later.
Owner
- Name: Rolf Fokkens
- Email: rolf@rolffokkens.nl
- Release notes owner:
Current status
Detailed Description
Recent Linux kernels support the use of Solid State Drives as caches for rotational hard disks. Because the high cost per GB for SSD devices this feature may bring the best of both: fast end big yet affordable storage capacity. Linux kernel 3.9 introduced dm-cache, kernel 3.10 introduces bcache.
Benefit to Fedora
Fedora will support SSD's in a way that brings the speed of SSD's to the user, while also bringing the capacity of HDD's. The user however does not need to plan what data to put where (SSD or HDD): the Linux kernel will take care of that based on the usage patern.
Scope
- Proposal owners: Enable caching features in new kernels
- Other developers: Support the caching features in their respective packages. Special focus should be on making the system boot from a cached root FS.
- Release engineering: All packages should operate in close harmony to make this work. Only a rebuild of the relevant packages is required.
- Policies and guidelines: No changes I think.
Upgrade/compatibility impact
Implementing a caching root FS using bcache requires a reinstall for sure, because bcache requires its own superblock on disks/volumes!
All existing Fedora functionality remains as is.
How To Test
Testing involves doing a clean install of Fedora, while configuring in anaconda a cached root FS. After the installation the cache state should be checked (if it is operational or not.
Additional tests:
- run benchmarks (see performance improvement over non cached storage)
- update the kernel (see if dracut builds a functional initramfs including caching
- ...
User Experience
Users should experience improved storage performance.
Dependencies
Although the kernel supports SSD caching, the other mentioned packages should be changed to support this and make it really work.
Contingency Plan
- Contingency mechanism: Revert the shipped configuration of the relevant packages.
- Contingency deadline: beta freeze
- Blocks release? No
Documentation
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/bcache.txt
http://forums.gentoo.org/viewtopic-t-959542.html
http://blog.kylemanna.com/linux/2013/06/30/ssd-caching-using-dmcache-tutorial/