Bug 1312371

Summary:	[RFE] blockdev: Allow setting cache mode of backing files (libvirt)
Product:	Red Hat Enterprise Linux Advanced Virtualization	Reporter:	Ademar Reis <areis>
Component:	libvirt	Assignee:	Virtualization Maintenance <virt-maint>
Status:	CLOSED DEFERRED	QA Contact:	yisun
Severity:	medium	Docs Contact:
Priority:	low
Version:	8.0	CC:	amit.shah, areis, bsettle, dyuan, hhan, juzhang, kchamart, knoel, kwolf, lihuang, lmen, mkenneth, virt-bugs, virt-maint, xuzhang
Target Milestone:	rc	Keywords:	FutureFeature, Improvement, Triaged
Target Release:	8.1
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	Enhancement
Doc Text:		Story Points:	---
Clone Of:	634658	Environment:
Last Closed:	2020-11-12 16:58:22 UTC	Type:	Feature Request
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Bug Depends On:	634658
Bug Blocks:	580953

Comment 1 Peter Krempa 2016-03-23 13:03:50 UTC

Summary from a private comment:

Description of problem:
Currently, when a snapshot is open with 'cache=none', the backing files are also opened without caching. However, as they are read-only, they can be cached. The downside is that it may cause duplication of caching - both in the host memory as well as the guest's. The upside is that if many VMs share the same backing file (as happens in VDI scenarios), this will make launching them much more efficient.
I think a configuration option allowing this would be helpful for VDI scenarios.

--- Additional comment from Kevin Wolf on 2016-02-26 10:46:25 BRT ---

Upstream commit 91a097e7 implements setting the cache modes for child nodes,
including the backing file. blockdev-add isn't a supported interface yet, but this can also be used with -drive and HMP drive_add.

Comment 4 Jaroslav Suchanek 2019-04-24 12:26:51 UTC

This bug is going to be addressed in next major release.

Comment 7 Jaroslav Suchanek 2020-11-12 16:58:22 UTC

This bug was closed deferred as a result of bug triage.

Please reopen if you disagree and provide justification why this bug should
get enough priority. Most important would be information about impact on
customer or layered product. Please indicate requested target release.