currently when you enter maintenance mode on a datastore, the vcenter will initiate i think 8 svmotions (storage vmotions) which takes quite a toll on our SAN. I’ve been following this thread: http://communities.vmware.com/thread/331892 which pointed me to this blog entry: http://www.boche.net/blog/index.php/2009/01/05/guest-blog-entry-vmotion-performance/
A Cold Migration has a cost of 1 and a Hot Migration aka VMotion has a cost of 4. I first set mine to 12 as I wanted to see if it would now allow 3 VMotions at once, I now permanently have mine set to 24 which gives me 6 simultaneous VMotions per Host (6×4 = 24).
by modifying the cost we can modify the number of svmotions that occur at once.
this article has good info: http://frankdenneman.nl/2012/06/limiting-the-number-of-storage-vmotions/
The operation cost of a Storage vMotion for datastores is set to 16, the max cost of a datastore is 128.
i set mine as
and this is allowing 4 hot svmotions.