dd is a native linux command also available on XenServer that can be used to measure the performance of a block device like /dev/sda at a block level. We can use this command to troubleshoot storage performance issues within the Host as well as the guest while clubbing it with other tools like iostat, vmstat

Instructions

Caution: Please use dd with care as interchanging the if and of can cause disk corruptions

1. Login into the XenServer Host having performance issue using putty

1. To measure read performance of a disk like /dev/sda, run the following command

             # dd if=/dev/sda of=/dev/zero

We can specify block size for read by adding the block size parameter bs

           # dd if=/dev/sda of=/dev/zero bs=1M

Sample output will be like

# dd if=/dev/sda of=/dev/zero

^C25577+0 records in

25576+0 records out

13094912 bytes (13 MB) copied, 1.46965 s, 8.9 MB/s

Here we can see it copied 13 MB at a rate 8.9 MB/s

To measure write performance of a disk , we can use the below command to write a file named testfile of 1 GB under /root which is on device sda

# dd if=/dev/zero of=/root bs=1M count=1024

Sample output like

dd if=/dev/zero of=/root/testfile bs=1M count=1024

1024+0 records in

1024+0 records out

1073741824 bytes (1.1 GB) copied, 12.9848 s, 82.7 MB/s

Here we can see it took 12 seconds to create a 1 GB file at 82.7 MB/s

In order to bypass the filesystem cache we can use oflag

# dd if=/dev/zero of=/root/testfile bs=1M count=1024 oflag=direct