Dynamic vs. fixed

Discussions related to using VirtualBox on Solaris hosts.

Dynamic vs. fixed

Postby TheLastBoyscout » 23. Jul 2011, 04:53

Hi there!

I was planning on installing WHS 2011 in VirtualBox running on SE11. The system sits on a mirrored rpool (2x 250 GB) and has a 4x 2 TB RAIDZ for storage and backup with SMB (classic "NAS"). I don't have the WHS ordered yet.

I assume I have to create two disks: One 160 GB system disk for the WHS and another disk as storage for the WHS (let's assume I want max. 2 TB for WHS, but I have no idea at the moment how much space I really need over the next year or so).
Now which option should I use when creating that data storage vdisk? Fixed or Dynamic?

The VirtualBox manual states: "While this format takes less space initially, the fact that VirtualBox needs to expand the image file consumes additional computing resources, so until the disk file size has stabilized, write operations may be slower than with fixed size disks. However, after a time the rate of growth will slow and the average penalty for write operations will be negligible."
But since the storage disk for WHS will continue to grow this is not really true in my case, or is it?
Since everything is stored on ZFS, doesn't a 2 TB sparse file make more sense in this case?

Does it even matter (in terms of file I/O performance and CPU load, I am running on an AMD E-350...)?

(first post, so bear with me... :mrgreen: )
-TLB
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Re: Dynamic vs. fixed

Postby kebabbert » 23. Jul 2011, 15:59

A fixed sized disk that is 100GB big, will create an empty file that is 100GB.

A dynamic sized disk that is 100GB big, will create a 0KB big file. As you save data into the disk, it will grow in size, until it gets 100GB. Max is 100GB.

I always use dynamic.
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Re: Dynamic vs. fixed

Postby TheLastBoyscout » 23. Jul 2011, 20:08

Thanks for your reply kebabbert!

I am familiar with the differences of fixed vs. dynamic at a high level, but was wondering whether the combination "fixed vdsk using sparse file" is better than dynamic vdsk since the disk size will never stop growing and thus I would be hit with the performance penalty mentioned.

I guess, it gets to the question where the file resize is more efficient? When VirtualBox does it or when the ZFS driver does it. Is there even a difference (i.e. there would be none if VirtualBox uses spare files for "Dynamic" vdsks on a Solaris host and than this is a non-issue)?

Thanks!
-TLB
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Re: Dynamic vs. fixed

Postby Perryg » 23. Jul 2011, 20:23

AFAIK there is a small hit in performance using dynamic, but after the disk/s start to fill the hit diminishes. IMHO the dynamic is the way to go for various reasons.
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