Bug 1373545

Summary:	Support of discard/fstrim
Product:	Red Hat Enterprise Linux 7	Reporter:	Pablo Iranzo Gómez <pablo.iranzo>
Component:	qemu-kvm	Assignee:	Ademar Reis <areis>
Status:	CLOSED NOTABUG	QA Contact:	Virtualization Bugs <virt-bugs>
Severity:	low	Docs Contact:
Priority:	low
Version:	7.2	CC:	chayang, juzhang, knoel, kwolf, qizhu, rbalakri, virt-maint
Target Milestone:	rc
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
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Last Closed:	2016-09-30 14:16:01 UTC	Type:	Bug
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Bug Depends On:
Bug Blocks:	1203710

Description Pablo Iranzo Gómez 2016-09-06 14:46:21 UTC

Description of problem:

While trying to create images for usage with 'packer' for a cloud environment, it has been found  tha the RHEL-base included KVM packages doesn't include the machine types that support discard/fstrim.

Updated machine types, like it was done on BZ 1229666 could help here.

Comment 3 Ademar Reis 2016-09-30 14:16:01 UTC

The discussion about machine types is a red herring.

Clarifying:

 - Discard/TRIM support was enabled by default in QEMU-1.5 upstream
 - Downstream, we ship QEMU-1.5.3 with RHEL-7.2
 - Which means yes, Discard/TRIM should work. The test-case verification is
   supposed to be simple: in the host, create the qcow2 (or even raw) image
   on a filesystem that supports sparse files (ext4 and xfs should both work);
   start a guest with this image and mount the fs with discard=on, then
   delete some files; the image size (space ocupied in the host fs) should
   decrease.

If this is not working, then we have a Bug somewhere. I haven't confirmed this personally because I don't have the setup ready.

Machine types enable the DISCARD feature, but this detail is only relevant if the VM is using an old machine-type (from RHEL6, for example, the VM was migrated from there or is reusing some old configuration).

Please *always* inform the qemu command line (from ps -auxw or from the libvirt logs) so we can understand what's being used.

Comment 4 Kevin Wolf 2016-09-30 15:01:57 UTC

(In reply to Ademar Reis from comment #3)
>  - Which means yes, Discard/TRIM should work. The test-case verification is
>    supposed to be simple: in the host, create the qcow2 (or even raw) image
>    on a filesystem that supports sparse files (ext4 and xfs should both
> work);
>    start a guest with this image and mount the fs with discard=on, then
>    delete some files; the image size (space ocupied in the host fs) should
>    decrease.

Please also make sure that discard='unmap' is set for the disk in the libvirt
XML so that the feature is actually enabled and discards don't get ignored.