Bug 589975 - RFE: virt-manager: allow setting cache mode for kvm/qemu disks
RFE: virt-manager: allow setting cache mode for kvm/qemu disks
Product: Fedora
Classification: Fedora
Component: virt-manager (Show other bugs)
All Linux
low Severity medium
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Assigned To: Cole Robinson
Fedora Extras Quality Assurance
Depends On:
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Reported: 2010-05-07 08:45 EDT by Tomasz Torcz
Modified: 2010-11-26 13:18 EST (History)
5 users (show)

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Fixed In Version:
Doc Type: Bug Fix
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Last Closed: 2010-11-26 13:18:52 EST
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RHEL 7.3 requirements from Atomic Host:
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Description Tomasz Torcz 2010-05-07 08:45:00 EDT
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 17:15:04 EDT
This is upstream now:

Comment 2 Tomasz Torcz 2010-11-26 13:18:52 EST
Indeed, virt-manager in F14 contains cache mode selector.

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