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Debian Drive transfer speed on VMware ESX

Posted on 2011-03-09
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Last Modified: 2012-05-11
Hi all

I have an unusual problem with Disk transfer speeds on a virtual Linux environment.

This environment consists of an ESX host running 3i, 3.5.0, 153875 and 2 Debian 5.0 Linux virtual machines.

When booting the machines have a hdparm transfer rate output of:

 /dev/sda:
 Timing cached reads:   1418 MB in  2.00 seconds = 707.95 MB/sec
 Timing buffered disk reads:  132 MB in  3.01 seconds =  43.93 MB/sec

After a couple of days, the machines become sluggish and the same test produces:

/dev/sda:
 Timing cached reads:   162 MB in  2.00 seconds =  80.99 MB/sec
 Timing buffered disk reads:   12 MB in  3.78 seconds =   3.18 MB/sec


The command that is being run is hdparm -tT /dev/sda

Any ideas on why this is occurring, I cant see anything obvious in the logs that will cause this.
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Question by:alexanderfoti
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by:bgoering
ID: 35083242
Is your vm configured with IDE drives?
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by:alexanderfoti
ID: 35083317
No its an ESX server, you are only able to choose SCSI Devices.
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bgoering earned 500 total points
ID: 35083467
Ah, I see it is 3.5 (you can use IDE drives on 4.1). I am unable to recreate this issue on 4.1 but I suggest you take a look at http://communities.vmware.com/message/1077902#1077902, and in particular if you have a vSMP vm try it with a single virtual cpu (or disable the page sharing as in http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1004901) On 3.5 the overhead of manageing vSMP was a bit higher than on 4.x, and sometimes one could realize a huge performance increade by reducing to a single vCPU.

Here are the results from one of my 2 vCPU mail servers that has been up for a while now on ESX 4.1:
[root@prodvmail01 ~]# hdparm -tT /dev/sda

/dev/sda:
 Timing cached reads:   2396 MB in  2.00 seconds = 1197.89 MB/sec
 Timing buffered disk reads:  496 MB in  3.01 seconds = 164.95 MB/sec
[root@prodvmail01 ~]# uptime
 08:51:40 up 773 days, 11:37,  3 users,  load average: 0.14, 0.08, 0.04
[root@prodvmail01 ~]#


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by:alexanderfoti
ID: 35084455
Many Thanks.

I have turned off VSMP on both boxes and have restarted them. Now all we have to do is wait until they slow down again now!

I have also removed more than one core from the virtual machines, so at the moment, they only have 1 core each.
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by:bgoering
ID: 35084951
Sounds good - keep us posted
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Author Comment

by:alexanderfoti
ID: 35093324
Bad news - HDparm results today:

/dev/sda:
 Timing cached reads:   1394 MB in  2.00 seconds = 695.98 MB/sec
 Timing buffered disk reads:    8 MB in  3.81 seconds =   2.10 MB/sec

Any ideas?
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by:alexanderfoti
ID: 35201391
After restarting the host, it appears the drive speeds have now returned to normal. Many thanksk
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