Re: [Xen-API] some confusion about SR and PBD and ...

[Tim <tim@xxxxxxxxxx>]

Hi Oliver,

> Hi to everyone !
> I'm new here, so this is my first post.
>
> I'm playing around with Xen Server on an Ubuntu server installation using LVM 
> as its main disk management framework. Well it's an installation option to do 
> so ... (maybe that wasn't a good idea to choose in the first place...)
> Now...
> Of course I came to the point where I needed an SR. Following some tutorials I 
> created a new partition, in my case an LV, and an SR on top of it.
> When I checked the LVM settings I saw that a new PV, VG and LV have been 
> created.
> So I thought "WOW" ... next I did was checking the data rate, so I wrote a file with 4G 
> in size (with dd) to the disk. One to the disk created for the SR and one to a disk one 
> "abstraction layer" lower (or obove ? ... whatever ... :) ).
> The difference was quite noticable ~175MB/s on the SR disk and ~211MB/s one 
> layer lower...
> Hm... I did some searching, but this seems to be the normal way...
>
> So my question: Is it somehow possible to attach an LV to be used (and managed) 
> by XEN as a SR more directly (instead of having it create this whole bunch of 
> virtulization-thingies...)
An LVM SR is basically a VG with corresponding LVs managed by the XAPI. 
That way XAPI can carve up the VG to provide VDIs to the domUs.
I don't think nesting PVs inside LVs causes much slowdown. Have you 
tried that on it's own to compare? If it is the nesting that's causing 
the problem, put your SR on a raw device (or partition).
I suspect it's blktap that is causing the slowdown. The domUs need to 
access the disk through dom0 somehow - through blktap. I don't think 
there's any getting away from that one.
I'd be interested to know what you find.

Kind Regards,

Tim

_______________________________________________
Xen-api mailing list
Xen-api@xxxxxxxxxxxxx
http://lists.xen.org/cgi-bin/mailman/listinfo/xen-api