Backup time varies from either 4.5 hours or 13 hours

[lmcintyre]

Hi.
My backup of 4 VM's sometimes works quickly, and other times doesnt:
Around 25% of the time, the whole backup will take 4.5 hours.
Around 75% of the time, the backup will take 13 hours.

It is always 1 or the other, and I cannot work out why. The server never has any extra load depending on the day of the backup, and the backups are full backups.

When it takes 13 hours, the time spent to backup is distributed along each VM (so it doesnt appear that there is a specific VM that is making it go slow).

Can anyone think of a reason why.. or where I should start looking as to why?

Any help appreciated!

[dellock6]

As always, first check job statistics about bottlenecks, if some values change between short and long job durations.
Also, consider CBT change of the VMs, are those VMs changing data inside their filesystems? Are you running scheduled defrags between job executions?