iSCSI physical vs. file extents

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sabreofsd

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Feb 8, 2012
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Hello all,
I was thinking about this as I was about to put up another FreeNAS box: Would it be better to use file (ZFS) extents rather than physical ones. My reasoning behind this is that if I used file extents, I could take advantage of caching and such. I've noticed with my physical extents, I don't use any RAM as a cache (as the disk access is just a pass through). As the new machine has 32G of RAM, I would love to enable this as a cache and the only way I see to do this is through a file extent. Thoughts?
 

survive

Behold the Wumpus
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May 28, 2011
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Hi sabreofsd,

I think I saw one of the devs say that on a high-end filer a file extent would be faster than a device extent. Assuming your observation that a file based extent can take advantage of the ZFS caching features that would certainly make sense.

That said, I would love for someone who actually knows what they are talking about to provide a little guidance.

-Will
 

sabreofsd

Dabbler
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Feb 8, 2012
Messages
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I might try to put together a test for this. The biggest problem I see is my network connection (gigabit). If I load balance multiple 1G connections, that should do the trick I hope.

I think that I have a spare 8 core box laying around that I can test this with. I'd also like to try it with something a little lower end to see what the results are.
 

survive

Behold the Wumpus
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Hi sabreofsd,

If my experience with iscsi is anything to go on maxing out a single gig-e link would be awesome! Running bonnie on a vm that resides on my iSCSI disk yields the following results:

Code:
              -------Sequential Output-------- ---Sequential Input-- --Random--
              -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine    MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU  /sec %CPU
          100 190638 85.7 39298 25.5  8277  3.4 46495 20.3 96962  9.8 912.0  4.7



I'm curious to see what you find out, so please update the thread with your results.

-Will
 
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