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From Logan Barfield <>
Subject Re: Libvirt & RBD caching
Date Wed, 18 Feb 2015 20:35:12 GMT
We're running FreeBSD with VirtIO.  Disk is VirtIO in the libvirt
config, and shows up as a 'vtbd' device in /dev.  I'm kind of stumped
on where the performance discrepancy could be coming from.

Thank You,

Logan Barfield
Tranquil Hosting

On Wed, Feb 18, 2015 at 3:23 PM, Wido den Hollander <> wrote:
> On 18-02-15 21:00, Logan Barfield wrote:
>> Our current deployment is KVM with Ceph RBD primary storage.  We have
>> rbd_cache enabled, and use "cache=none" in Qemu by default.
>> I've been running some tests to try to figure out why our write speeds
>> with FreeBSD are significantly lower than Linux.  I was testing both
>> RBD and local SSD storage, with various cache configurations.  Out of
>> all of them the only one that performed close to our standard Linux
>> images was local SSD, Qemu cache=writeback, FreeBSD gpt journal
>> enabled.
> I think the main problem is that FreeBSD uses ide/scsi and Linux uses
> VirtIO.
> Have you tried running FreeBSD 10 with the OS type set to Ubuntu for
> example?
>> I've been reading on various lists the reasons and risks for
>> cache=none vs cache=writeback:
>> - cache=none: Safer for live migration
>> - cache=writeback: Ceph RBD docs claim that this is required for data
>> integrity when using rbd_cache
>> From what I can tell performance is generally the same with both,
>> except in the case of FreeBSD.
>> What is the current line of thinking on this?  Should be using 'none'
>> or 'writeback' with RBD by default?  Is 'writeback' considered safe
>> for live migration?
> None should still be ok, since librbd does the caching. It's not Qemu
> who does the caching.
> Wido

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