[DRBD-user] Slow Reads on VM - Xenserver and DRBD

Christian Balzer chibi at gol.com
Tue Jul 24 06:14:52 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Mon, 23 Jul 2012 17:52:21 +0200 Phil Stricker wrote:

> Hi!
> I just started with a setup of DRBD and Xenserver 6.0.2
> Of course, my read-performance inside the VMs is very bad. 

Of course why? Did you expect a bad read performance and if so, what made
you believe so?

>The storage
> system is a Raid 10 with 8x Intel 320 SSD on an LSI-Raid-Controller.
What LSI controller? I can get better performance than your 477MB/s with a
software RAID 10 and just 5 plain 2TB Hitachi SATA drives...

What is the speed of the raw backing device? 
For reads, it should be same as the DRBD device...

> Testing the Read-Performance with hdparm on Dom-0 shows:
> /dev/drbd1:
> Timing buffered disk reads:  1432 MB in  3.00 seconds = 477.18 MB/sec
You should be able to get much better numbers than that, also depending on
your controller a RAID6 using 8 disks can be faster (in all aspects!) than
a RAID 10 with 8 drives. Simply put, more active data disks to read from
and write to. Of course the controller needs to have a fast enough CPU to
do the checksum crunching. 

> HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate
> ioctl for device
> If I start the same test in a VM, I get:
> Timing buffered disk reads:  100 MB in  2.56 seconds =  39.10 MB/sec
> There is just one single VM on that system.
> Can you help me? Windows VMs have shown the same issue. 
That clearly looks like some Xen deficiency or configuration issue, can't
help you with that, sorry.

> The problem does
> also consist, if the second node is disconnected...
Reads are ALWAYS local and should not be affected by the presence of
replication or not.


Christian Balzer        Network/Systems Engineer                
chibi at gol.com   	Global OnLine Japan/Fusion Communications

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