Vcenter Clustering\ LUN's access

[dcs115]

This may be a silly question but my normal thought process with a LUN (Windows and Linux thinking), is never present it to more than server at a time, as if both are reading and writing to it, you can get corruption.

Clustering with ESX seems to depart from that theory, as if you create a datastore from a LUN, you can scan for it on all the hosts in your cluster and have different hosts store VM's there (and VMotion etc). I am assuming this is just ESX magic, but i wanted to make sure i wasnt missing something.

Thanks!

David

[TobiasKracht]

You can write data to LUN from two servers at one time only in case of using clustering FS.

StarWind Software R&D

[Brahell]

There are a explanation of how to VMware ESX works with SAN in a following documentation (page 32):

The key is VMFS that manage access across ESX hosts.

Brahell

[AndreTheGiant]

The key point is the difference between filesystem NOT designed for clustering and filesystem cluster-aware.

VMFS (but also GFS for example) is a filesystem designed for clustering: you can share the SAME LUN between multiple node in a active-active (on the specific LUN) configuration.

NTFS (but also ext2/3 for example) is a "traditional" filesystem: you cannot share and use the same LUN at the same time from different node cause you can have metadata corruption. The only solution is use application cluster (like MSCS) to share the same LUN in a active-passive (on the specific LUN) configuration.

Andre

[GixxerDave]

DCS115,

The cluster software of Vcenter takes care of who is connected to the LUN. Even thought all nodes can see the LUNS the cluster is in control of making sure there is no corruption. Microsoft clustering is the same, the cluster software controls who is attached to a LUN.

Make sure you create a cluster in Vcenter and add nodes to it. On the SAN side give access to all nodes to the "shared" LUNS.

Another way of saying this is, the LUNS are virtualized and the nodes go through the cluster of Vcenter to get access to LUNS. So nodes do not directly connect to LUNS, they must go though the virtual layer first...