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From "Ronald van Zantvoort (JIRA)" <>
Subject [jira] [Commented] (CLOUDSTACK-1302) Add per storage setting for cache="none/writeback/writethrough" options for VMs on KVM hypervisor
Date Mon, 31 Aug 2015 11:52:46 GMT


Ronald van Zantvoort commented on CLOUDSTACK-1302:

IMHO This feature is implemented in the wrong place.

The cache setting is a vital tunable which determines QEMU's cache behaviour and it's proper
setting is both based on how much risk you wish to take with your data and the combined settings
of the backend stores you're connecting to.
For example, Ceph will demand writeback for safe RBD's cache operation, while for an NFS or
a battery-backed RAID you'd might want to resort to none or even writethrough (since the caching
can reside on another level).

The Disk Offerings have the explicit ability to resolve to different kinds of Stores which
might/will need different settings. It therefore makes no sense to put it there; it'd merely
reflect a quite likely dangerous preference. Whether or not anyone would *want* it to be a
*preference* is another question.

Personally I believe Cloud Providers should never try to play fast & loose with customers'
data (and there's already plenty of ways to do that anyway, even if the setting sits with
the Stores), and customers wishing to take it easy on their transaction safety have a plethora
of options of doing that within their VM.

> Add per storage setting for cache="none/writeback/writethrough" options for VMs on KVM
> -------------------------------------------------------------------------------------------------
>                 Key: CLOUDSTACK-1302
>                 URL:
>             Project: CloudStack
>          Issue Type: Bug
>      Security Level: Public(Anyone can view this level - this is the default.) 
>          Components: Management Server
>    Affects Versions: 4.0.0
>         Environment: Ubuntu 12.04.2 Host, KVM hypervisor
>            Reporter: Jason Villalta
>            Assignee: Wido den Hollander
>            Priority: Minor
>             Fix For: 4.4.0, 4.5.0
> Version 4.0.0 of Cloudstack has a hard coded value of cache=none for virtual machines
deployed.  This causes conflict with filesystems mounted with fuse such as ZFS, GlusterFs
and CEPH as fuse does not support directio with these file systems.  When starting VMs libvirt
throws an error "could not open disk image <disk> Invalid argument"
> Changing the cache= setting to writethough or writeback solves this problem but there
currently is no way to set this.  Ideally this would get set on a per datastore basis.

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