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I want to use SAN storage as shared disk for two active nodes running on Linux, and I want to use it as raw device (without file system).

At first I thought it would have no problem if the disk is visible to both nodes. But while searching I saw that concurrent access from both nodes to shared storage is not available unless global file system (such as GFS) is used. (as informed in Can a SAN allow concurrent write access to the same file from several servers?). I have some further questions associated with my situation.

  1. What is the scope of the lock which is attained by a node when it tries to write on the shared device? Does it get a block lock? or each disk (LUNs) get a lock?
  2. Is it necessary to have clustering s/w like VCS or CLVM to achieve active-active cluster with SAN? Is there any alternative ways?
  3. I think function of clustering softwares mentioned above is scheduling I/Os using lock distribution. Is it enough to support active-active cluster?
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From SAN/Storage perspective, you can map a LUN to several hosts without restrictions. But, it's a duty of host, OS and applications (typically it's clustered software/volume manager/filesystem) to take care of where and how to read/write data on the shared device in order at least not to:

  • Write data to the same blocks simultaneously
  • Read data from the block of the disk that was modified by another host asynchronously so updates are still in the OS/filesystem cache on that other node, but not on the disk itself

But as you are going to use raw devices you or your application already know how to avoid such deadly collisions. If not, you should look at an available for you cluster solution.

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