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Please can you help me compare disk types for vmware server?

I know "independent nonpersistent" is slower. But what about "independent persistent" versus "dependent" (or how do you call this one?)

What about raw disks (direct access to physical disks)? And I found I cannot change the type from independent to dependent later? Is it possible to create a dependent raw disk when starting a new vm?

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2 Answers 2

Hands down, "Raw Disks" is direct access to the physical disk and by far the fastest access and disk I/O, known as RDM. The downside is you cannot move the VM to another machine without imaging the physical disk that you mapped to that VM.

But, you asked which is the fastest "Virtual Disk". RDMs are not virtual disks.

And no, you cannot switch between virtual and raw types (unless I am missing an upgrade or something). EDIT: I think there are tools now that let you convert a physical disk to a VMDK. As for converting a VMDK to raw disk, I think you can just copy the contents to the physical drive (as long as it isn't a boot drive), and attach it normally to your VM.

As for which virtual disk is faster? Fixed size is much faster than that dynamic type, which has a cost of expansion. Fixed disk is close to speeds as Raw (as long as you aren't doing sequential writes with a database), but still much faster than dynamic.

Now, if you are asking if it makes a difference to break the files into chunks or have one single large file - there are many discussions on the net that go both ways. Personally, having them split up into multiples I feel increases your host OS' seek time as it must go and find all different files, especially if you defragged your local drive.

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Thank you, and sorry for my unclear question, which I now corrected. I still wonder if (a) "dependent" type is possible for raw disks. And if (b) "independent-persistent" is faster or "dependent" for virtual disks. –  Anonymous Apr 9 '09 at 11:19
    
I actually do not know the different of those types of virtual disks. :) –  Eric Duncan Apr 9 '09 at 18:54
    
Thank you very much for all the details, this is helping me a lot! –  Anonymous Apr 1 '10 at 21:49

Hands down, "Raw Disks" is direct access to the physical disk and by far the fastest access and disk I/O. The downside is you cannot move the VM to another machine without imaging the raw disk.

----> I assume by raw disks you are talking about RDM but you should check http://www.vmware.com/files/pdf/performance_char_vmfs_rdm.pdf. Most of the time, RDM are not performing better than VMDK files.

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@kagemaru I disagree (quoting my original answer, and yes I was talking about RDM = Raw Device Mapping). This does not answer the original question either, as he was asking for comparisons. If you have ever done performance metrics on Relational Database Management Systems (aka "RDBMS") then you'll immediately see a performance drop in sequential writes. Even directly quoted from that PDF you linked to, "For sequentialreads and writes, performance of VMFS is very close to that of RDM(except on sequential reads with an I/O block size of 4K)." This is where it falls down fast, very fast. –  Eric Duncan Jul 15 '13 at 14:53

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