Re: [Xen-users] xen disk io performance

[George Shuklin <george.shuklin@xxxxxxxxx>]

use flashcache.

In my tests it works perfectly (about 15% degradation compare to 
fileio/wb mode for iscsi, which I accept as highest possible (but risky) 
storage access mode).
On 29.11.2011 18:10, Jeff Sturm wrote:
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
> > bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Alexandre Chapellon
> > Sent: Tuesday, November 29, 2011 4:27 AM
> >
> > Le 29/11/2011 04:14, George Shuklin a Ãcrit :
> > > "Very high" and "very slow" sounds like "i pay many buks but got small
> > > potion".
> > >
> > > How many IOPS postgre generates from dom0 point of view? (see
> > > statistics in vbd/tap device in /sys). How do you did you check netapp
> > > storage performance?
> > I have found I have queries that are putting the system on its knees, Some are
> > statistics collection and adding the right index just solved the problem. I 
> > never looked in
> > /sys on dom0 to get informations about iops. Instead I used iostats in domU and 
> > it
> > gave me ~ 5000 read/s when the statistic collection.
> > If I compare to what I see in /sys on the dom0 (looking in the stat of the dm- 
> > block
> > device , not the underlying devices) I see ~ 1200 write/s and 100 read/s when 
> > thing
> > are ok.
> Mechanical disks are slow.  The rules for disk performance haven't really 
> changed due to virtualization: lots of RAM, lots of buffering.
>
> In other words, avoid disk accesses like the plague.  Especially random I/O to 
> physical drives.
>
> Solid-state storage helps, but you can often achieve the same effect with lots 
> of RAM, on the cheap.
>
> I've fought similar issues on our virtualized clusters.  After many cycles of 
> tuning and monitoring, I came to a couple of conclusions.  One, a 16-disk 
> storage array isn't enough for 30 guests--despite plenty of capacity and 
> bandwidth, random I/O is still the problem, which can only be mitigated with 
> more spindles (we'd normally have at least 2 per physical host, but in our 
> virtual cluster we've allocated one-fourth of that).
>
> Two, rotating media are 1970's technology, good for little more than archival, 
> ripe for replacement.  I'm keeping an eye on price/capacity for solid state 
> storage.
>
> One little tip for Linux users:  Mount guest file systems with "noatime" 
> whenever you can.   You'll be glad you did.
>
> -Jeff
>
>
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users