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Adding a file system to a raw device of course takes up some overhead.

The question is how many percent does ZFS take?

And does overhead differ depending on the ZFS raid set?

Update

The purpose is to be able to be able to estimate usable disk space when designing a NAS.

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closed as not a real question by MDMarra, ThatGraemeGuy, Magellan, Scott Pack, Ward Nov 5 '12 at 21:41

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

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How much overhead, compared to what? –  ewwhite Nov 5 '12 at 19:33
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There aren't very many use cases for data on raw devices, so a filesystem is almost necessary. If you have a specific case, you should mention it, like Oracle databases on raw devices vs Oracle databases on ZFS. –  MDMarra Nov 5 '12 at 19:36
    
@ewwhite Updated the post. it is for designing NAS purposes. –  Sandra Nov 5 '12 at 19:46
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2 Answers

up vote 4 down vote accepted

The disk space lost in formatting a volume with the ZFS filesystem is negligible. If you set up mirrored vdevs or RAIDZ/RAIDZ2, then you lose the diskspace for the parity bits or mirrored data.

This article walks you through the options.

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The most significant "overhead" will be the performance hit seen on very full pools:

Keep pool space under 80% utilization to maintain pool performance. Currently, pool performance can degrade when a pool is very full and file systems are updated frequently, such as on a busy mail server. Full pools might cause a performance penalty, but no other issues. If the primary workload is immutable files (write once, never remove), then you can keep a pool in the 95-96% utilization range. Keep in mind that even with mostly static content in the 95-96% range, write, read, and resilvering performance might suffer.

From the ZFS best practices guide

Some ZFS features like ditto blocks or checksums may further increase redundancy and thus decrease the net capacity, although the exact impact will greatly depend on the configuration of your pool.

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