Revision as of 20:27, 8 April 2014

LVM Cache Logical Volumes

Summary

LVM can now use fast block devices (e.g. SSDs and PCIe Flash) to improve the performance of larger but slower block devices. These hierarchical or layered logical volumes are called "Cache Logical Volumes" in LVM.

Owner

Name: Alasdair G. Kergon
Email: agk@redhat.com
Name: David Cantrell
Email: dcantrel@redhat.com
Name: Dave Lehman
Email: dlehman@fedoraproject.org
Release notes owner:

Current status

Targeted release: Fedora 21
Last updated: (DATE)
Tracker bug: <will be assigned by the Wrangler>

Detailed Description

LVM is now capable of using fast block devices (e.g. SSDs) as write-back or write-though caches for larger slower block devices. Users can create cache logical volumes to improve the performance of their existing logical volumes or create new cache logical volumes composed of a small and fast device coupled with a large and slow device. These cache logical volumes can be used with most LVM segment types, including RAID 1/4/5/6/10, linear, stripe and thin pools.

Benefit to Fedora

Users will have the advantage of fast block device speeds with the capacity of larger but slower block devices.

Scope

Proposal owners:

Other developers: N/A (not a System Wide Change)

Release engineering: N/A (not a System Wide Change)

Policies and guidelines: N/A (not a System Wide Change)

Upgrade/compatibility impact

Nothing changes for users following update path.
The logical volumes using cache will become unavailable from systems using older kernels and LVM. The cache will have to be dropped to grant access for those legacy systems.

How To Test

It is not strictly necessary to have SSDs or other fast block devices. You can use any block device as the cache for any other block device - even if they are the same device, as long as there is enough space! While you may not see the benefits of this arrangement without a fast device, it is still possible to check for correct functionality (i.e. can the cache LV be created, removed, resized, renamed, or changed? is there any corruption? etc).

If Anaconda has support for LVM cache logical volumes, users will be able to configure them at install time. Otherwise, it is possible to convert existing logical volumes to cache logical volumes once the system has been installed.

Information on how to setup cache logical volumes can be found in the lvm(8) man page.

User Experience

Users will enjoy the increased speed of disk storage while keeping capacity of HDD.

Dependencies

Anaconda: Implementation of UI
Dracut: Module to Detect and install required configuration files and kernel modules

Contingency Plan

LVM2: Changes are already present upstream. If severe bugs are found, cache can be disabled here.
Dracut: There is already a hardcoded dm-cache installation. If the detection is not done in time, it will work as is.
Anaconda: Cache will work even without UI except it will not be configurable by regular users without using command line. Users can also setup cache LVs after installation.
LVM2: We will need an ability to drop the cache should the cache device fail or become corrupted. (This can be done using Live CD.)

Contingency deadline: beta freeze
Blocks release? No
Blocks product? -

Documentation

Command line: Cache section of the upstream [lvm(8)] man page.

Release Notes

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-{{admon/important | Comments and Explanations | The page source contains comments providing guidance to fill out each section. They are invisible when viewing this page. To read it, choose the "edit" link.<br/> '''Copy the source to a ''new page'' before making changes!  DO NOT EDIT THIS TEMPLATE FOR YOUR CHANGE PROPOSAL.'''}}
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--->
-<!-- The actual name of your proposed change page should look something like: Changes/Your_Change_Proposal_Name.  This keeps all change proposals in the same namespace -->
 = LVM Cache Logical Volumes <!-- The name of your change proposal --> =
 == Summary ==
-<!-- A sentence or two summarizing what this change is and what it will do. This information is used for the overall changeset summary page for each release. -->
 LVM can now use fast block devices (e.g. SSDs and PCIe Flash) to improve the performance of larger but slower block devices.  These hierarchical or layered logical volumes are called "Cache Logical Volumes" in LVM.
@@ Line 63: / Line 39: @@
 == Detailed Description ==
-<!-- Expand on the summary, if appropriate.  A couple sentences suffices to explain the goal, but the more details you can provide the better. -->
 LVM is now capable of using fast block devices (e.g. SSDs) as write-back or write-though caches for larger slower block devices.  Users can create cache logical volumes to improve the performance of their existing logical volumes or create new cache logical volumes composed of a small and fast device coupled with a large and slow device.  These cache logical volumes can be used with most LVM segment types, including RAID 1/4/5/6/10, linear, stripe and thin pools.
 == Benefit to Fedora ==
-<!-- What is the benefit to the platform?  If this is a major capability update, what has changed?  If this is a new functionality, what capabilities does it bring? Why will Fedora become a better distribution or project because of this proposal?-->
 Users will have the advantage of fast block device speeds with the capacity of larger but slower block devices.
@@ Line 104: / Line 78: @@
 . What are the expected results of those actions?
 -->
-<!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
+It is not strictly necessary to have SSDs or other fast block devices.  You can use any block device as the cache for any other block device - even if they are the same device, as long as there is enough space!  While you may not see the benefits of this arrangement without a fast device, it is still possible to check for correct functionality (i.e. can the cache LV be created, removed, resized, renamed, or changed?  is there any corruption?  etc).
-To test the feature user will have to:
-* install the system with cache configured once the Anaconda UI is available
-* converting existing LVs to use SSD cache once the LVM package is updated to include latest upstream patches
-An SSD or other fast block device is recommended to actually increase the speed but the feature will be testable without one as the user may create a cache device from the data disk with effect of decreased performance.
+If Anaconda has support for LVM cache logical volumes, users will be able to configure them at install time.  Otherwise, it is possible to convert existing logical volumes to cache logical volumes once the system has been installed.
 Information on how to setup cache logical volumes can be found in the lvm(8) man page.

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