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UseLunReset and UseDeviceReset VMware


If you are using a SAN attached ESX environment, make  sure:
Disk.UseLunReset is set to 1 (default = 1)
Disk.UseDeviceReset is set to 0 (default = 1).

The reason to disable the Disk.UseDeviceReset param is because it does a complete SCSI bus reset. All SCSI reservations will be cleared, not for a a specific LUN but for the complete device (being the whole SAN controller).

This could disrupt your SAN fabric. I would suggest setting the ESX host in maintenance mode and reboot it afterwards.

Alternatively, you can also set this via the Service Console by issuing the following commands:

esxcfg-advcfg -s 1 /Disk/UseLunReset
esxcfg-advcfg -s 0 /Disk/UseDeviceReset
service mgmt-vmware restart

Tuning ESX(i) for better storage performance


Many applications are designed to issue large I/O requests for higher bandwidth. ESX/ESXi 3.5 and ESX/ESXi 4.x support increased limits for the maximum I/O request size passed to storage devices. These versions of ESX pass I/O requests as large as 32MB directly to the storage device. I/O requests larger than this are split into several, smaller-sized I/O requests.

Some storage devices, like the EVA, have been found to exhibit reduced performance when passed large I/O requests (above 128KB). As a fix for this, you must lower the maximum I/O size ESX allows before splitting I/O requests.

To reduce the size of I/O requests passed to the storage device using the VMware Infrastructure/ vSphere Client:

  1. Go to Host > Configuration.
  2. Click Advanced Settings.
  3. Go to Disk.
  4. Select Disk.DiskMaxIOSize.
  5. Change value to 128.