[pve-devel] Fwd: Re: recommanded cache setting for rbd image

[Dietmar Maurer]

> For me, we should have same write performance with cache=none or
> cache=writeback.
> but with writeback we need to flush from guest + host. (overhead ?).

From 'man 2 open':

      O_DIRECT (Since Linux 2.4.10)
	Try to minimize cache effects of the I/O to and from this file.  In general  
	this  will degrade performance,

 
> Maybe some block driver like rbd can benefit from writeback on host, as they
> can aggregate writes from differents guest, and flush them once ?
> Maybe same for iscsi devices, the host scheduler can reorder writes from
> differents guest and flush them once ?
> 
> 
> I'll so some benchs today, cache=none vs cache=writeback, with netapp
> nfs,nexenta iscsi and local sas drives with raid controller with write cache.

That would be great. Let's see if there is really any difference.