Operations that require getting a file lock or a metadata lock in VMFS result in short-lived SCSI reservations. SCSI reservations lock an entire LUN. Excessive SCSI reservations by a server can cause performance degradation on other servers accessing the same VMFS.

Examples of operations that require getting file locks or metadata locks include:

Virtual machine power on.


Virtual machines running with virtual disk snapshots.

File operations that require opening files or doing metadata updates.


ESX/ESXi uses the SCSI reservations mechanism only when a LUN is not VAAI capable. If a LUN is VAAI capable and supports Hardware Acceleration, ESX/ESXi uses atomic test and set (ATS) algorithm to lock the LUN.

Performance degradation can occur if such operations occur frequently on multiple servers accessing the same VMFS. For instance, VMware recommends that you do not run many virtual machines from multiple servers that are using virtual disk snapshots on the same VMFS. Limit the number of VMFS file operations when many virtual machines run on the VMFS.