The udev cache was used for obtaining a device list, but this sometimes returned an incomplete list which led to missing volume errors. Therefore, creating several disks in parallel could fail.
LVM disabled the udev cache in the in RHEL 6.5, eliminating this error. However, on machines upgraded from earlier versions, lvm does not change the current configuration after an upgrade. It is the responsiblity of the system adminstrator to check the new lvm configuration and update the old confguration with new values.
The udev cache is now disabled in vdsm lvm configuration, which ensures proper configuration also on upgraded machines.