Re: [Xen-users] xen disk io performance

[Jeff Sturm <jeff.sturm@xxxxxxxxxx>]

> -----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 écrit :
> > "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



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