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SCSI Bus Sharing on SCSI Cards

Recently working with a monster VM I had a request to add more capacity after looking at the properties of this machine I noticed that it had almost reach the maximum number of possible hard drives (SCSI devices) that could be allocated due to the amount of capacity and RDM’s required.  So based on this I thought I would write a blog post on why there is this limitation.

Now lets just take a quick look at the wide SCSI bus which is capable of serving SCSI ID 0 to 15 which means 16 SCSI targets. As you know (if you’ve dealt with SCSI) traditionally SCSI ID 7 is reserved for the SCSI card itself which then leaves us with 15 SCSI disk targets and with the maximum number of 4 SCSI adapters allowed in a VM this could become an issue especially in the earlier versions of ESX/vSphere datastore sizes.

On this VM the first SCSI card (SCSI0:x) is set for VM Boot Disk plus  a number of others and that has its SCSI Bus Sharing set to NONE which leaves the remaining three SCSI adapters set for RDM’s as VIRTUAL or PHYSICAL.

SCSI Bus Sharing is explained in the following overview:

Option Description
None   Virtual disks cannot be shared by other virtual machines.
Virtual Virtual disks can be shared by virtual machines on same server.
Physical Virtual disks can be shared by virtual machines on any server.


S
o typically we have:scsi_bus1

With the above you can quickly see how you can use up the number of disks assigned to a VM ( 3 x 15 = 45 devices) plus your Boot Disk

Now the question is could I set the first SCSI controller to Virtual or Physical instead of None thus allowing me to increase the number of drives and therefore increase capacity.  After some research I found out that VMware do support this scenario but with a caveat that once enabled you could effectively share the Boot Disk which if accidently shared with another VM could cause possible data corruption  🙁 – yes humans can make mistakes ……

So now we have

scsi_bus2

As you can see it is possible to increase the number of drives but I would be very careful if you decide to implement this.

So with Monster VM’s becoming more popular you could have the ability to add extra space but I  would not personally recommend this.

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