Questions about Affinities.

[JesusAli]

So when using Xsan Admin to create my Xsan Volume, I set Affinities for two Storage Pools, VIDEO (4 LUNs) and OTHER (1 LUN).

Then I used Xsan Admin to make two folders on the Xsan Volume. "VIDEO" and "SCRATCH". I gave Video Affinity to Video, and gave Scratch Affinity to Other.

So that means when I write a video file to the VIDEO folder, it specifically gets saved to the VIDEO storage pool, right?

Well, what I'm wondering is, how does the Mac OS know what types of data to send to which Affinities?

I mean, I named it "video" but how does the Mac OS know what kind of data is video?


For instance, when I run AJA System test, is there a way to specifically target the Video folder or better yet, the VIDEO Storage Pool?

When I use Xsan Tuner, the only Affinity I can choose is Metadata and Journaling? I don't know why that is the case. It's the only Affinity Tag I didn't specifically set.


I'm not sure I'm very coherent here, but if anyone has some light to shine, it would be appreciated.

Thanks, Jesus

[MattG]

Great questions.

Firstly, Xsan is not part of the Mac OS, at all. It's a filesystem only, and not unique to Apple. Xsan's filesystem is the same as Quantum's StorNext filesystem, which can run on Linux (and other Unix variants) and Windows as well.

So, the reason that Xsan "knows" which Storage Pool it should place your file is based on rules that come into effect after the write command escapes the OS and makes its way to a low level program that addresses a kernel extension that asks the metadata controller to write the file. The metadata controller looks at the tag you placed on the folder earlier and then tells the client machine to write the file to a particular storage pool.

Unfortunately, programs like AJA System Test don't offer a good test of Xsan performance for two reasons. One has to do with the issue you mention: you can't direct the test file to a particular location. This means that you can't dictate which Storage Pool to test in volumes with multiple sotrage pools. The AJA System Test also fools people into thinking that their system is underperforming because it only burns the bandwidth needed for the particular test you ask for.

Xsan Tuner is also problematic because it was never rewritten as a Universal Binary and therefore behaves erratically on Intel Macs.

You may then ask, "what tools are available for testing my Xsan's performance?" and the answer is, not much, at least not much in the GUI-based department. I'll let others chime in as to what they use.