So either an RDM or a VMDK file. Ths explains it. ESX 3.5 can use only a single connection to the volume. I assume you don't have 10 Gbps network? This means that the max throughput to the SAN is 128 MBps. Depending on the type of disk IOPS will be quite lower. In my experience, if you are using SATA disks, you will see around 900 IOPS. SAS disks will get you around 4000 IOPS.
That local SCSI controller will likely hit 300 MBps and 2000 IOPS. So your large sequential IO will be quite slower as will your IOPS.
We remedied this in a few ways. Use SAS disks in the SAN when connecting the host to the SAN. Use an iSCSI initiator within the VM and use multiple vNics to acheive more than 1 Gbps throughput. Or upgrade to ESX4. ESX 4 lets you multipath to the SAN. I have seen huge performance increases with this feature alone. I do not have numbers yet as it is only in our test lab with a lower performing SAN. But that lower performing SAN outperforms our production SATA SAN under ESX 3.5 U4.
Charles Killmer, VCP
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Charles Killmer, VCP4
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