Re: [User question] Huge buffer size on KVM host

[Avi Kivity <avi@redhat.com>]

On 08/16/2012 05:54 PM, Martin Wawro wrote:
> 
> On Aug 15, 2012, at 2:57 PM, Avi Kivity wrote:
> 
>>> 
>>> We are using logical volumes and the cache is set to 'none'.
>> 
>> Strange, that should work without any buffering.
>> 
>> What the contents of
>> 
>>  /sys/block/sda/queue/hw_sector_size
>> 
>> and
>> 
>>  /sys/block/sda/queue/logical_block_size
>> 
>> ?
>> 
> 
> Hi Avi,
> 
> It seems that the kernel on that particular machine is too old, those entries are
> not featured. We checked on a comparable setup with a newer kernel and both entries
> were set to 512.
> 
> We also did have a third more thorough look on the caching. It turns out that the
> virt-manager does not seem to honor the caching adjusted in the GUI correctly.
> We disabled caching on all virtual devices for this particular VM and checking
> with "ps -fxal" revealed, that only one of those devices (and a rather small one too)
> had this set. We corrected this in the XML file directly and the buffer size
> currently resides at around 1.8 GB after rebooting the VM (the only virtio device
> not having the cache=none option set is now the (non-mounted) cdrom).
> 

cc += libvirt-list

Is there a reason that cdroms don't get cache=none?


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