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VMFS block size

The filesystem designed by VMware can use different block size during the datastore initialization. Each size defines some limits.

For more information see: VMFS block size


One interesting aspect of VMFS3, is the sub-block allocation as described in KB 1003565: sub blocks are used for directories and small files with size smaller than 1 MB. When the VMFS uses all the sub block (4096 sub blocks of 64 KB each), file blocks will be used. For files of 1 MB or higher, file blocks are used.


But the number of 64K sub-blocks is limited, reached this limit also small files or directory entries will use "full" blocks.



Finding the sub-block limit

Seems that this limit is not documented (or not too clear documented), so how many sub-blocks are available?

One way could be try to debug VMFS metadata or from the first 20MB of each datastore partition (see for example: or or by using the linux VMFS "driver" (

Seems that "usual" vmware commands (like vmkfstools) do not give this info (but I've not tried too much).


So I've tried a simple approach to find this limit: just make a lot of small file and see when they do not use sub-block size.

In my case I've used ESX (just to have the COS and full Linux commands) and for each volume I've used those commands:

cd /vmfs/volumes/DATASTORE_NAME
echo "Find the number of sub-blocks currently used"
du -ah | grep "^64K" | wc -l
echo "Find the number of directory in the datastore"
ls -l | wc -l
echo "Start building small files"
mkdir delme ; cd delme
for FILE in $(seq 1 3999); do dd if=/dev/zero bs=1k count=1 of=$FILE 2> /dev/null; du -sh $FILE; 

After than, remember to remove the "delme" folder.


The results are interesting:


datastore1 - 100 existing sub-blocks - 15 dirs

64K     3845

1.0M    3846


datastore2 - 212 existing sub-blocks - 27 dirs

64K     3721

2.0M    3722



So seems that for each datastore the magic number (by adding also the number of directory) is 3960.

Block size seems that does not change this number (datastore1 is 1MB and the other is 2MB).

But probably the number is just a coincidence (due to the usage of only 2 datastores), because I've repeat the same test on some local datastores, and the number isn't the same (but it's always really near this value).

Not sure if minor version of VMFS3 could change this value.




The number of sub-blocks seems reasonable for a VMFS3 datastore.

Usually the number of folders and small files not not too big, because the number of VMs per datastore is limited (and for each VMs the number of related files is also limited).

This mean than using large block size is not a serius problem in disk usage.

...Go back to Part 7...



What else can I say? Probably the VCDX certification is not for all... it require a lot of time, a lot of experience, some "stress", some travels, is not only a technical exam, it requires a lot of skills.

But complete this path give a lot of satisfactions... and seems that also people that has not pass the last step are in favour of this this kind of path.


PS: sorry for my (bad) english, but it's not my native language


Some questions

Official FAQs -


Q: How many VCDX's are there?

A: The VCDX official list is not updated in real time (usually there is a delay of about one month or more), so the answer could be on tweeter or on last posts of


Q: How much time does it take to become a VCDX?

A: There isn't a minum or maximum time. You need experience and you must be already a VCP. Then it can take 1 year (like in my case, for Feb 2009 to Feb 2010) or more or less (I had to wait a lot until the design exam was available).

The last part is the one more time consuming because you have to realize your application and submit some months before the defense.


Q: Is still possible to reach the VCDX3 certification?

A: Yes, but only for few days... The last defense opportunity seems to be the PEX 2011, and the application deadline is the Nov, 22 2010!


Q: It is possible to reach the VCDX4 certification?

A: Not yet! The VCAP-DCD beta period is finished, but the exam is not yet avable. Also for this reason there aren't yet any date for the VCDX4 defense (and PEX 2011 deadline is to close). So first new VCDX4 (from the new path) will probably be avable from Q2 or Q3 of 2011.

But VCDX3 that have passed the DCD beta exam will become the first VCDX4.


Q: What about VCDX5 certification?

A: Probably it will take more time, VCDX4 was announced on May, 2010... about one year late the release of vSphere products. Actually the new vSphere beta period has started (to be honest is near to start at the end of Nov, 2010). Actually is not clear it this will be the new 5.x series (maybe it could remain a 4.5)... So until a new 5.x product is not released (with the VCP5 certification), is quite difficult to talk of think about VCDX5


Q: You must be a "master" of big environments to become a VCDX?

A: Really not... You must be able to design a good and complete solution. This sentence of Duncan's blog is very clear about it: "Show you can design for large scale. It’s OK if your actual project was for a small environment, but show that you can think big too."


Q: You must be an active member of the VMware community to become a VCDX?

A: There isn't a correlation... Some VCDX do not partecipate to the community, but can be really usefull partecipate to learn more (and of course to help other community people).


Q: You must be an active blogger to become a VCDX?

A: There isn't a correlation... Several VCDXs are famous bloggers. But other famous bloggers are not VCDX...

Panel is fair-minded with candidated...


Q: You must be active on twitter to become a VCDX?

A: There isn't a correlation... But in the last period more information are on twitter rather than on community... so start follow some right people



Part 1 - Introduction

Part 2 - Pro and cons

Part 3 - The application

Part 4 - The interview

Part 5 - Other info on defense

Part 6 - Admin exam

Part 7 - Design exam


The end?

...Go back to Part 6...


The Design exam

The Design exam of VCDX3 was probably the weak part of the entire path.

The suggested course does not cover the exam at all (but to be onest was useful for the application part).

The exam itself maybe was not completly oriented on the design aspect, but there were also some questions about administration, installation, and so on...

If found it also not so difficult to pass.


Anyway the exam is no more avaible, and the new one (for VCAP-DCD and VCDX4) seems a lot better:

Just lets see when it goes live to see also other experience about it.



Old Design exam resources


VCAP-DCD resources

Blueprint -

Demo of the Visio part -


Other user's experience in Design exam

Old Design exam


New DCD exam (actually only the beta version)


Read more in Part 8...

...Go back to Part 5....


The Admin exam

This is the part that requires a lot of practical experience... This was the first reason that I've tried this path… to prove myself my technical knowledge.

And I think that this is the right way to do a technical exam… You must demonstrate your technical capacity… not your memory capacity.


On the VCDX3 path this exam was (on October 2010 it was retired) composed by a part of close questions (very deep on some technical arguments) and a part that is a real environment with some tasks to complete.

The second part is very similar to some kind of Cisco exams, or some Novell Linux exams (like CLP) or what was (as I know is not more available) Microsoft practical exam on Active Directory (83-640).

The most interested aspect is that you have a full system (not a limited simulator) so you have all commands and also the help on it!

The advantage is that your have to wait some weeks to know the results… This part can be improved, because Novell CLP and Microsoft 83-640 exams were able (I do not know in which way) to give the results after the exam.


For the VCDX4, this exam is the VCAP-DCA exam and now (it is available from October 2010) is based only on practical tasks (so no more the close questions part)… so is a very technical exam!

Unfortunately I was not able (due to other commitments) to participate at the beta period… and now I want first to do the VCAP-DCD exam (and only I have some spare time or a closest center also the DCA).



VCAP-DCA resources

Blueprint -

Lab Demo -

Exam Center Locations -


Other user's experience in Admin exam

Old Admin exam


New DCA exam


Read more in Part 7...

...Go back to Part 4....


Other user's experience in the the defense/interview


Other user's experience in the application


Other user's experience of people that have not passed (yet)

Probably there are also other links, but I cannot find them. As written the pass rate is quite low (less than 50%), but also unsuccessful stories can help very much.

Maybe an improvement can be put in the feedback of people that have not pass the interview to explain more how and where the candidate can improve. There are some indication of area that must be improved, but maybe too much generic.


Other sources

Blogs are not the primary sources of information of VCDX paths.

There is also the VMware community  (but seems not too much active on VCDX area), twitter (I think that could be must, for example to find other candidates in the same period) and more important a direct iteraction with people that are in same boat or that have already had this experience (and events like VMworld could be a good chance to meet them).

Note that NDR do not permit at VCDX or previous candidate to talk about the questions or the design details, but seems to be permited (as written also in the official Defense Prep) that a VCDX make a review of your design ("Consider that VCDX-certified architects are reviewing your application" is the exact sentence).


Read more in Part 6...

...Go back to Part 3...


The interview

You will know if the application has passed a “formal” check after a week. But you have to wait more (in my case a month) if has passed also a technical review.

If  the application has been approved it means that is good enough? I’m not sure about this, but this certainly means that you can go to last phase: the defense!

The defense is a part of the interview step, that is an on-site meeting with a panel of VCDX certified architects… you will find people with a great and deep knowledge, with more experience, and more skill… a big challenge!

Panel is usually composed by three VCDX from VMware (in my case: Duncan Epping, Pang Chen and Andrew Hald), but also other peole may be in the room with you: one (or more) from VMware Certification staff and maybe there can some observers (also from VMware).


This step is the more expensive one (compared to the others) both for the fee and the travel expensive. Unfortunately there are only few panel windows at specific places in one year… So just plan your defense period with enough time.

A good choice is choose VMware events, like VMworld or Partner Exchange (in my case was the PEX 2010 and was also a way to see Las Vegas).


As written in the prep guide the interview process consists of three parts.

  • The VCDX defense (75 minutes), with a brief introduction of your project (15 min), where you will answer questions from panelists about the design you submitted and you will have an opportunity to provide further insight into your design decisions and rationales.

  • The design scenario (30 minutes), where you will demonstrate that you can begin to work through design issues in a methodical and logical manner.

  • The troubleshooting scenario (15 minutes), where you will demonstrate that you can begin to work through design or operational problems in a methodical and logical manner. With only a limited time!


What about the language? This could be a problem, because now you must talk and understand a conversational English (more different that simple read and write) and you cannot use dictionary or spell checker...

I have no much occasions to speak English (I try to listen it by following some series in native English), this could be another reason to take this step during a VMware event, just to prepare for the language…

But I have to assert that the panel was very king on this aspect, and I was also lucky to find people with a good, slow and clear English. Great thanks to Duncan and Melissa that have hearten and tranquillize myself during the break.



The defense could be very simple, if you have a good and clear design… in this case you have only to support and argue it.

But you have to know it very well… Seems strange, but after a month (or more) you can forget some aspect or decision about your project!

You have enough time to a good presentiation (you can use PowerPoint or similar tools) of your project... use it in the right way... explain also some choices.

Then there are a "lot of" time to the discussion and the defense... be prepared, build some extra slides (if you think that could be useful to explan some choices), keep your Visio and your diagram close at hand...

I think that in this step you can also save a not well designed project… But I’m not sure about this.

I’m only sure that you can damage a good design with a worse defense!


After this phase you will get a little break... If your design was on vSphere and you are on a VCDX3 certification, at this point you have also to remember to "reset" yourself to VI3.5, because next parts is on this version.



The design require a good experience and a good approach: you cannot produce a complete design in only 30 minutes… so use different approach… try to cope with all the problems and requirements… use an high level approach and, when needed, a deeper one.

As written in the prep guide: The focus is on the journey to the solution. Talk and ask as you can and think out-loud. And for VCDX3 remember that is will be VI 3.5 design... know the limitation of this product and forget vSphere!

Consider the panel also as the customer (with a good skill)... you have to explain and argue your choices... but you have also ask a lot to get all the information that you need for the desing. And take a look at the countdown... time is not enough... use it in the right way!

Note that you have to know (at higher level) also storage enterprise, networking, applications... not stricly related to VMware.



The troubleshooting IMHO is the big challenge of the interview, because you have a very limited time to try to find the solution of the problem. I think that the way and the approach that you use could be considered positively also if you do not find the solution. Again talk and ask as you can and think out-loud.

In this case the Panel act as a customer and you must act as a technical support... The scenario (IMHO) is similar by a phone call support so you have to ask information, ask to make test, ask for the result of it, and so on...  And take a look at the countdown... time may be not enough... use it in the right way!

There is a solution or is simple a "[Kobayashi Maru|]" test? I'm quite sure that a solution exist because (in my case) the problem was a real custom problem.


Other comments

A recent post of Duncan Epping give more hints on the defense process:

Some interesting note is available also in the VMware Community, for example in this thread: (go on the last page).

Remember also that pass ratio in this phase is quite low: less than 50%, and in some cases very low (for example on Munich 2010 defense on 2 people pass (on 12 candidates, if I remember right).


Read more in Part 5...

...Go back to Part 2....


The last step of a long journey

I will start from the last step of the VCDX path, because I think that is the most interest and hardest part, but also the more distintive part.


As clearly documented in the application form ( this final step is structured in two distinct phases:

  • the application and project submission

  • the inteview with the project defense

Those phases have also two different and specific fee.


Probably is also the more difficult part, as the pass rate is lower than 50%. How each part of the application and the interview compose the final result (that is only a pass or not pass) is not clear (for me), but I suppose that the application and the defense are the most important part.

Also I do not know if the results of Admin and Design exams are used to build the final results.


The application

When I done my application, there wasn't any information on how make it, only the information in the application form itself.

Now some good infos are available in the preparation guide:

And also the Design Workshop V4 can help a lot, because you can use the material as a track of your design or as a way to do a review.


As written in the prep guide, the application process is similar to submitting a resume for a job.

It is composed by the application form (filled in all its parts) and a project, which could be real or fictitious, but must be realistic and “well designed”.


For VCDX3 is possible submit a vSphere design. I think that could be better submit a vSphere design, at least you can reuse it if you cannot complete the VCDX3 path and you go directly to the VCDX4 one.

About vSphere 4.0 or 4.1 I'm not sure, but I suppose that both could be fine... But just explain why you will choose one or the other...


I think that (IMHO) this is the hardest step of the entire VCDX path, because there isn’t a template on what you have to produce and there are only few information or suggestions about this part.

This is certainly the longer part (in term of required time), because at least 30-40 hours are required, but more are better and more are required for non english native people.


How many pages is not clear and is not specify at all… I found that some projects have a size between 100-250 pages, but I have no idea if it’s the right size. The content is more important! Also it must cover all the point of the applications and maybe more

Mine was about 10 documents and about 150 pages... so not so big.


How many documents or files also is not specify, but I suggest to structure the documents in a clear and simple way, in a form similar as you would submit to a customer.

As written in the prep guide, mandatory VDCX documentation are:

  • Architectural design document with diagrams and blueprints

  • Implementation and next steps documentation

  • Validation/test plans

  • Operational plan/guide

  • Installation and configuration instructions


What about the language? If english is not your first language (as my case) try to keep the sentence simple and shot. And use a good spell and grammar checker

Remember also to check that the names of technologies and products are written in the right way (for example: VMware and not Vmware!).


There is a template for your project? Not officially, but if you have access to Partner Network you can find some template for standard virtualization project. This could be a starting point… not for the content, but for the structure of your documents.


Some other hints? I'm under NDR, so I can't say more... But see VCDX Tips from VCDX 001 John Arrasjid:


This step does not require any travel and can be done also before other steps, you have only the deadline related to your defense sessions. Pay attention that when you receive the application's  “invite” you will have probably not so much time until the deadline… So prepare your application before!

When you have finished the application you have only to send the zip file via email and wait…

There will be a first reply (if there is any formal problem in the application) and usually after 1-2 weeks there will be a first technical rewiev... if passed then you can go to the next and final step!


Read more in Part 4…


Note that if the application is accepted, this does not mean that is good enough to pass the defense... The application must be complete in each part... but I do not know people where the second review (the first is formal, the second is technical) was refused (but this does not mean that was never occured). So be very careful on this part... Be sure to submit a good application to be able to have a good defense on it!

Now it's official... I've no luck with VMware beta exam...


For the beta of old VCDX3 Design exam, I was not selected... but this could happen...

For the beta of VCAP-DCA exam I've got no time to do it due to a work week and a holiday week... Then the exam period was extended, but I never got a notifice of it... so I miss this this opportunity...

The beta for VCP4 was taken on 14 Jul with a hot weater, without air-conditioner in the room, with the sun in back of me, and without water!!! But this was a fault of the testing center... I can only say that was hard to anwer at the huge amount of questions in those condictions.


But those issues are nothing compared on what has appen on VCAP-DCD beta exam day.

Also this time the beta period overlap a week with heavy working week, and this time the beta period duration was only one week... so the only way was to take the exam on Friday.

As all the VCAP/VCDX exams my country does not have any VUE center... so I have to travel by fly (all center are far more than 500km).

The only option with fly booking was Frankfurt am Main, but the available flights near me are on Hahn Airport (call from Ryanair Frankfurt-Hahn, but really far from Frankfurt)... I see the flights schedules, the bus schedules, and so I've booked the exam at 11:30 am. Obviously during the booking, my credit cards were not accepted so I had to book by phone


Now the day of the exam... Flight was 8:35 -> 9:55 and on time... But I do not found the bus scheduled on 10:00... I found only a cold and rainy weater and a new bus schedule

With new schedules I lost more than 3 hours (including the return to the airport, to get the flight to home), and this was a disaster... not enough time to take the exam.

I've called the VUE center, to notify my delay and the problem... but there was no solution.

After 1.45 of bus travel I arrived at central station, and then quickly to the center at 12:30.

At this point, after registration and the short initial pool of the exam I finally start the exam!


Now the exam... The big penalty was that I've got less than 3 hours (because the bus will leave before 4:00 pm), instead of the near 4 hours + 30 min of extra time for not english native people.

It was a nightmare answer at the questions with a lot of hurry, without the time to review the answer and without the back button... For this reason on some questions I've accidentally push the next button without give an answer


The exam itself was quite better than the previous design exam (althought some questions seems similar).


About the positive aspects:

  • Most the questions are good... But can be improved (I expected some questions and arguments that weren't present)

  • Good balance on differenct aspects (But, and is strange, some aspects are covered - I'm under NDR so I cannot tell which )

  • Visio designs (yes... now are more design to do) are good and really better than before


About the negative aspects:

  • No back button and no way to do a final review... this is probably a fault only for the beta, but for me is very important (expecially the review, very useful for me, that I'm not a navitve english, when I'm find that I'm loosing too much time to read or translate a question).

  • Less time for 131 questions, and some of them are very time expensive (like the visio diagram). But this is probably a problem only for the beta exam.

  • More design (and this is correct), but some still question too much relater to operations to do... Maybe could be better move this kind of question the DCA exam, but is only a my opinion.

  • As several people have remark there are some questions the require a knowleger on hardware vendor's model... I agree that could be necessary, but I suggest to give at least the option of the 3 large vendors, or balance the questions on the different vendor.

  • In at least question some acronym are relevant to the answer, but for example those acronym are not used on my country... not really nice.

  • For a couple of question the content of the vSphere Design course is (I think) required to give the right answer... But the course itself is only suggested and the material is not public available... So this could be little problem.

  • I've do not found a clear way to remove the connector in the Visio part... Only by draggin a connected object to the trash... But is not intuive at all

  • Maybe some sentences were not clear (or I've not not undestood them in the right way)... But the the clock that was running, I've got no time to analyze them


At 15:28 I finished the exam (less then 3 hours and unfortunately I gave a limited feedback, due to time constraints and due the impossibility to gave a general feedback at the end).

Then quickly to the bus and than more 2 hours to wait my flight that had also a delay (only 15 min, to be onest).

After landed a bus to the parking and then finally home.


A day to leave behind and a bad feeling on the exam results



To read other experience have a look at this site:


To some hints on exam see those threads:

any news on DCD exam date?

DCD Beta Exam Experience

Starting from ESX 4.0, the "legacy" version of VMware's bare-metal hypervisor is a real VM, with also a virtual disk (by default named esxconsole.vmdk).

Unfortunately this does not mean that is more flexible, because you cannot manage it from vSphere Client.


But there a several (unsupported) way to manage this virtual disk:


How to Move the COS (esxconsole.vmdk) in VMware ESX 4 (vSphere)


Adding a second VMDK to your ESX Service Console


How to extend the virtual disk of the Service Console

How to extend the virtual disk of the Service Console


ESX fails to boot when the disk containing the datastore with esxconsole.vmdk is detected as a snapshot


Partitioning your ESX host – part II


Best Practices for vSphere (ESX 4) Service Console Partitions