While performing a SAN-based backup to an IBM MegaRAID Repository, which in turn is mounted to the vRanger physical machine as a local drive, the write speeds of the backup appear excessively slow. This is especially noted if the Datastore LUNs being read from are located on the SAME IBM SAN hardware as the Repository LUN.
Possible causes may be:
1) Reading and writing from/to the same physical SAN with a single fibrechannel adapter.
2) IBM MegaRAID SANs ship with the physical disk cache of the drives within the array disabled.
To remedy the above:
1) If possible, use a second fibre adapter in the vRanger physical machine, and zone it in such a way that it is how the Repository LUN is mounted. Then, remove the presentation of the Repository LUN from the original fibre adapter. This will force READ traffic to come into the vRanger machine on one card/bus connection, and WRITE traffic to the Repository LUN to go out through the newly-installed secondary adapter. This provides better data-flow and use of cache on the fibre adapters themselves, and in turn lessens windows caching/paging operations, increasing overall performance.
2) Using the MegaRAID configuration utility (from the BIOS of the card during bootup), enable the Disk Write Cache setting, as well as setting the Write Cache on the RAID controller to use the type "Write Back", if applicable (NOTE: Write Back cache enable on the MegaRAID card requires a cache battery-backup module to be installed on the card. Otherwise, set the cache type to "Write Through").