xentop VBD_RD & VBD_WR output

Question

I'm writing a perl script that track the output of xentop tool, I'm not sure what is the meaning for VBD_RD & VBD_WR

Following http://support.citrix.com/article/CTX127896

VBD_RD number displays read requests

VBD_WR number displays write requests

Dose anyone know how read & write requests are measured in bytes, kilobytes, megabytes??

Any ideas?

Thank you

Answer 1

As far as I understood, xentop shows you two different measuers (two for read and write).

VBD_RD and VBD_WR's measures are unit. The number of times you tried to access to the block device. This does not say anything about the the number of bytes you have read or written.

The second measure read (VBD_RSECT) and write (VBD_WSECT) are measured in "sectors". You can find the size of the sectors by using xenstore-ls (https://serverfault.com/questions/153196/xen-find-vbd-id-for-physical-disks) (in my case it was 512).

The unit of a sector is in bytes (http://xen.1045712.n5.nabble.com/xen-3-3-testing-blkif-Clarify-units-for-sector-sized-blkif-request-params-td2620172.html).

So if the VBD_WR value is 2, VBD_WSECT value is 10, sector size is 512. We have written 10 * 512 bytes in two different requests (you tried to access to the block device two times but we know nothing about how much bytes were written in each request but we only know the total). To find disk I/O you can periodically check these values and take the derivative between those values.

I suppose the sector size might change for each block device somehow, so it might be worthy to check the xenstore-ls output for each domain but I'm not sure. You can probably define it in the cfg file too.

This is what I found out and understood so far. I hope this helps.