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4 Drives: ZFS raid setup advice please

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TunaMaxx

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Nov 29, 2011
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Hello! I'm new to FreeNAS, but I'm really impressed with my first build. It's time to do a second NAS and I could use some advice on setting up the ZFS pool.

I have four 2TB WD20EARX drives, going into another HP Proliant N40L MicroServer with 8GB of RAM and an Intel EXPI9301CTBLK GB NIC.

This is a backup server, but not mission critical. Nobody will die if data is lost. :)

What are the advantages/disadvantages of

1) 3 disk raid-z1 with hot spare
2) 4 disk raid-z1
3) 4 disk raid-z2

I ask, because I remember seeing mention about performance benefits with odd-numbered raid-z1 configs, but I can't find the info anymore.

If you were in my situation, what setup would you choose?
 

survive

Behold the Wumpus
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May 28, 2011
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Hi TunaMaxx,

I don't think that hot spares really work right now, so you could toss that option. Of the remaining 2, since it's "just" a backup server I would go for capacity so I would do raidz.

Since it sounds like you have everything already I would try them both & see if you see any performance differences....or if the performance boost is worth a quarter of the capacity.

-Will
 

sunflashx

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Sep 13, 2011
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Pretty much any form of disk subsystem will be adequate for servicing a 1gbps file share.

In my case, I staggered my disk purchases between a couple resellers and a month or so of time to try and get different batches so hopefully they don't all fail at once. Samsung also does Advance RMA, so I can get a replacement disk in my hands fairly quick with nothing out of pocket. Being able to sustain a single disk failure is a risk I'm comfortable with.

I'd find out what your warantee process will be. I'd hate to be remotely dependent on the box and be driven to run out and buy a replacement disk at full price because you can't get a replacement in a reasonable amount of time.

I wouldn't do a spare unless you had more than one array in the pool (assuming it works in the first place). Otherwise you may as well have it included in the array.

I vote for a RAIDZ1.
 

TunaMaxx

Dabbler
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Nov 29, 2011
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Thank you for the replies.

I don't think that hot spares really work right now, so you could toss that option.
Hot spare doesn't work? Oops... What doesn't work? I was able to set it up easily on the first NAS project, but never tested it. Does that mean I have a drive sitting there... idling away... that will never jump in when needed?

...I staggered my disk purchases between a couple resellers and a month or so of time to try...
As for the advice about buying drives from different vendors at different times... well that would have been smart. However, I did grab FIVE drives so I would have one kicking around just in case. :)

Looking like it's a four disk raidz1 then, huh?
 

Milhouse

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Jun 1, 2011
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I recently did some basic disk performance tests on an N36L with 4GB RAM and 4x1TB Samsung drives, comparing RAIDZ1/RAIDZ2, 512B and 4KB sectors, with and without read ahead (vfs.zfs.prefetch_disable=0):

Code:
WRITE     RAIDZ1          RAIDZ2
512B      212,143,286     148,067,931
4K        211,027,799     145,136,615

READ      RAIDZ1          RAIDZ2
512B      134,556,578     101,815,843
4K        191,722,812     108,278,443
512B+RA   417,207,180     320,201,198
4K+RA	  422,350,077     341,636,028


Enabling read ahead (RA) - predictably - had no effect on write performance, but significantly improved read performance.

Write performance reduces in line with additional parity overhead, but even with RAIDZ2 it is still more than enough for a single GigE connection.

Using 4K sectors seems to be a good solution even with non-Advanced Format drives - it has no obvious impact on writes, but does improve read performance.

So which redundancy level to choose? Either will suffice for a backup server over a GigE link, so performance will be a non-issue, in which case it all comes down to your capacity and availability needs.

If 4TB of storage is sufficient for your needs, I'd go with RAIDZ2 - no point risking data loss (even if it's not important) for storage capacity you don't need.
 

TunaMaxx

Dabbler
Joined
Nov 29, 2011
Messages
16
I have to commend you all. I've never been treated so well as a noob on any forum. The responses so far have been great; thank you.

I recently did some basic disk performance tests on an N36L with 4GB RAM and 4x1TB Samsung drives, comparing RAIDZ1/RAIDZ2, 512B and 4KB sectors, with and without read ahead (vfs.zfs.prefetch_disable=0):

Looks like I have some testing to do on my own once all the parts arrive. Thanks for the nudge in the right direction, and especially the RA tip!
 
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