Changes/SSD cache

SSD cache

Summary

Using recent kernel (3.9 and later) features for (fast) SSD caching of (slow) ordinary hard disks.

Owner

• Name: Rolf Fokkens
• Email: rolf@rolffokkens.nl
• Release notes owner:

Current status

• Targeted release: Fedora 20
• Last updated: 2013-07-08
• Tracker bug: <will be assigned by the Wrangler>

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 te 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/device-mapper/cache.txt

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/