At the VMworld 2010 Krishna Raj Raja and Haiping Yang have delivered a great session called Troubleshooting using ESXTOP for Advanced Users (TA6720). This session contains the new esxtop features which can be found in ESX 4.1. One of the new metrics is RESV/s (SCSI reservations per second). Duncan Epping has also written an article about it called Did you know? SCSI Reservations
VMFS is a clustered file system and uses SCSI reservations as part of its distributed locking algorithms. Administrative operations, such as creating or deleting a virtual disk, extending a VMFS volume, or creating or deleting snapshots, result in metadata updates to the file system using locks, and thus result in SCSI reservations.
Reservations are also generated when you expand a virtual disk for a virtual machine with a snapshot. A reservation causes the LUN to be available exclusively to a single ESX host for a brief period of time. Although it is acceptable practice to perform a limited number of administrative tasks during peak hours, it is preferable to postpone major maintenance or configuration tasks to off‐peak hours in order to minimize the impact on virtual machine performance.
In this video I’ll show you the new SCSI Reservation stats and the impact on the storage performance when the number if SCSI reservations becomes too high.
The information in this video is not completely correct, you can listen for an explanation in the VMware Communities Roundtable Podcast number 118 with Krishna Raj Raja.
