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I wish I could retract that comment about adding:- at the point the seek is done, you need to wait for the platter to spin into place, on average half a turn (the latency time). They do therefore happen in sequence! (If it was not for those pesky disks always spinning, I’d have been right!). Lesson learned: don’t let intuition lead you astray, don’t blog in haste, and realise that sometimes oft repeated advice is true. I am keeping the post up out of intellectual honesty, but will be blogging furiously to get it off the front page.
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I have just read an article on roughly how many IOPs you can expect from a single disk and encountered what I consider to be a frequently repeated mistake in the calculation.

Before I begin I want to point out that it is only an approximation anyway and caching in enterprise storage systems makes it perhaps a moot point anyway.

The article is here if you want to go and see it.
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Posted by tom under VMware & rant & storage | No Comments »

On Saturday I took and passed the VCP410 exam to get VCP4.

It was not really that hard, though I have been reading about vSphere since before it shipped, follow loads of blogs on VMware, installed it as soon as it came into beta and migrated my companies clusters to it relatively early. I would say if you have VCP3 and have used vSphere you should be OK.

The frustrating thing about the exam was the questions on the config max document, in my view if you are approaching the maximums you could just look it up and memorisation is a pointless exercise. A lot of the maximums are just decisions someone in vmware made, how many NFS stores by default ? (8), max? (64). What it the tree-depth per resource pool? (12… unless you use DRS, then it’s 10). This kind of memorisation is stupid, pointless, hoop-jumping and will be the difference between passing and failing for lots of people.

The exam (like most IT certs) is multiple choice so the questions are fairly mundane and of course there is only 1 correct answer. When interviewing candidates, I always prefer questions that start “what is your” rather than “what is” as anything that is so unsubtle as to only have one answer is probably too uninteresting to spend time discussing.

I did do a nights worth of revision however, using:

• Vmware’s mock exam
• Simon Longs practice exams
• The config max doc – YAWN
• My own mind map (using FreeMind, or use the pdf version) of config max and a few of Simons questions
• vReference’s card

Also worth considering are

• Trainsignals videos (I may still get myself a copy)
• The Exam Blueprint
• The VMware Doc’s
• vReference’s vSphere4 notes (these look excellent but I did not have time to read them)
• Mike Laverick’s book VMware vSphere 4 Implementation (I have it pre-ordered)
• Scott Lowes book Mastering VMware VSphere 4 (seems to be the most popular one released so far)

You may like to see the things I have added to delicious on vmware over the last few years.

Good luck if you take it too!

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Posted by tom under VMware | 2 Comments »