Why has VMWare ESXi 5 slow access to IBM M1015 / LSI 9240-8i RAID?

Question

I am building server for virtualization and wanted to go with VMWare ESXi 5. I configured RAID10 on 4 disks connected to internal RAID controller IBM M1015 (identical to LSI 9240-8i) and installed ESXi without any problems. Only problem is that copying data to datastore (on the RAID array) on host is slow - around 20MB/s. And I get about the same speed when trying to copy data to shared folder on guest virtual machine. Host network is autonegotiated 1000Full with 1000Mbit switch and I used vmxnet3 vm card in guest machine.

When I install MS Hyper-V on the exact same server with the exact same RAID10 array I get speed around 110MB/s when copying data to the Hyper-V host.

I used latest available drivers for every system:

ESXi 5 - LSI_5_34-455140.zip\scsi-megaraid-sas-5.34-1vmw.500.0.0.406165.x86_64.vib

Windows - 5.2.112

I even updated card firmware to 20.10.1-0077 .

This card is entry level but recommmended on different places for ESXi. What am I doing wrong? Am I missing something?

Solution

I got explanation from guys on VMWare forum in thread http://communities.vmware.com/message/2012333 . And the reason is that ESXi does no caching and completely relies on the controller for writing data to disks so speed 20MB/s with write-through cache mode (only mode this controller supports as it does not have BBU) is standard. Hyper-V is apparently doing some caching on OS level (which is not so safe on power failure/system crash) and so it can achieve much higher speed. Other controller with write-back cache should be able to achieve about 90MB/s.