[Xen-users] Slow IO performance

[Tomas Florian <tomas@xxxxxxxxxx>]

Hello,

I'm having trouble with slow IO performance under Xen 2.0.7 with 2.6 
kernel.  I'm running 3 physical machine.  I did hdparm -tT in my Dom0 on 
both servers and this is what I get:
Celeron 2.4 GHz  with 80 GB Maxtor UDMA6
Timing cached reads:   2160 MB in  2.00 seconds = 1080.00 MB/sec
Timing buffered disk reads:   10 MB in  3.56 seconds =   2.81 MB/sec

Celeron 2.4 GHz with
160 GB Seagate UDMA2
Timing cached reads:   272 MB in  2.00 seconds = 136.00 MB/sec
Timing buffered disk reads:    6 MB in  4.49 seconds =   1.34 MB/sec
60 GB Maxtor UDMA6
Timing cached reads:   448 MB in  2.07 seconds = 216.43 MB/sec
Timing buffered disk reads:    6 MB in  4.18 seconds =   1.44 MB/sec

AMD 800 MHz with 80 GB Seagate UDMA5
Timing cached reads:   556 MB in  2.00 seconds = 278.00 MB/sec
Timing buffered disk reads:    6 MB in  3.33 seconds =   1.80 MB/sec


As you can see both of these are quite slow - even 2.81 MB/sec is pretty 
horrible performance ... and it shows in my iowait which is 100 for long 
periods to the point of crashing (which is why I need to fix this 
problem).  I was also wondering if hdparm is giving me the right results 
... but I verified that they are in the ballpark by running a ramdisk -> 
disk dump and measuring the time.   My XenU's gets similar results.
For reference - on similar hardware without Xen I get 20 MB/sec on the 
old AMD and 50 MB/sec on the Celerons ... so more than 10x better.  I 
also have a P4 with HT and a fast SATA hard drive where I get 20 MB/sec 
but that's under VMWare 5 ... one of the reasons I picked Xen is that 
supposedly Xen should be much more efficient at IO than VMWare .. I'm 
guessing that without VMWare that particular drive would run at around 
65 MB/sec so VMWare loses about 70% (what I'd guess from the 
benchmarks)  ... my xen seems to be losing up to 98% of the original 
(that's pretty horrible!)
Any idea what could be going on?  Is this normal?  Is 3.0 any better in 
this regard? Also could it be related to my BVT settings? 

I generally use the following settings
xm bvt Domain-0 1 0 0 0 0
xm bvt VeryImportant 1 0 0 0 0
xm bvt SomewhatImportant 10 0 0 0 0
xm bvt NotImportant 1000 0 0 0 0

Thank you,
Tomas


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