Bug 589975 - RFE: virt-manager: allow setting cache mode for kvm/qemu disks
Summary: RFE: virt-manager: allow setting cache mode for kvm/qemu disks
Alias: None
Product: Fedora
Classification: Fedora
Component: virt-manager   
(Show other bugs)
Version: 13
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Cole Robinson
QA Contact: Fedora Extras Quality Assurance
Depends On:
TreeView+ depends on / blocked
Reported: 2010-05-07 12:45 UTC by Tomasz Torcz
Modified: 2010-11-26 18:18 UTC (History)
5 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of:
Last Closed: 2010-11-26 18:18:52 UTC
Type: ---
Regression: ---
Mount Type: ---
Documentation: ---
Verified Versions:
Category: ---
oVirt Team: ---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

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Description Tomasz Torcz 2010-05-07 12:45:00 UTC
Description of problem:
On some filesystems (btrfs) default caching in qemu makes guest extremely slow. It could be improved greatly by changing caching to writeback mode. The cost of this change: degraded reliability in case of host crash.
Please provide drop-down list to change the mode in Disk details view in virt-manager.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Go to disk properties.

Actual results:
No dropdown list for cache mode.

Expected results:
Cache mode: dropdown list with options "writethrough (default)", "writeback", "none (Direct_IO)".
Nb. I'm not sure if "none" really equals dio.

Additional info:

It is possible to manually edit guest .xml file to change caching mode. This change is retained by virt-manager and provides requested performance. Example:

# grep -C2 cache /etc/libvirt/qemu/scratch-F13-32b.xml
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='writeback'/>
      <source file='/mnt/storage_ST_250G/vms/scratch-F13-32b.img'/>
      <target dev='vda' bus='virtio'/>

the "cache='writeback'" part was added manually.

Comment 1 Cole Robinson 2010-05-27 21:15:04 UTC
This is upstream now:


Comment 2 Tomasz Torcz 2010-11-26 18:18:52 UTC
Indeed, virt-manager in F14 contains cache mode selector.

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