Which raid should I choose?

[Yamanipanuchi]

I have 8-500gb drives I want to setup a little home storage with. Freenas suggests 2 groups of RAIDz2 to be optimal. RAIDz3 gives me a slight more disk space.

After scouring for any additional Pro's or Con's I couldn't find any. I'm sure it all comes to performance, but is there way to get an idea before hand as to what performance differences I will see?

The system will mostly be used for Photo's and a few movies here and there. Important stuff, But nothing I wouldn't have 2nd, 3rd and maybe 4 drive at other families houses for backup.

AMD Phenom X4
8gb RAM ECC

 

[eraser]

I found this blog post a while ago and it was useful for me. I don't think the basics have changed much since it was written.

http://constantin.glez.de/blog/2010/06/closer-look-zfs-vdevs-and-performance

 

[anodos]

I prefer striping four mirrored vdevs over striping two 4-disk raidz2 vdevs. Resilvering mirrors is faster and growing the zpool in the future is imho simpler.

 

[cyberjock]

I prefer RAIDZ2s over mirrors because if you lose a single disk you have lost redundancy for a vdev. All you need is for that one disk to fail and you have major problems. On the other hand, you must lose 3 disks in one RAIDZ2 vdev before you lost your data.

 

[anodos]

I prefer RAIDZ2s over mirrors because if you lose a single disk you have lost redundancy for a vdev. All you need is for that one disk to fail and you have major problems. On the other hand, you must lose 3 disks in one RAIDZ2 vdev before you lost your data.

True, I guess RAIDZ2 makes more sense in a home environment where the general rule is that people don't do proper backups and are more likely to leave problems unaddressed or unnoticed.

 

[cyberjock]

True, I guess RAIDZ2 makes more sense in a home environment where the general rule is that people don't do proper backups and are more likely to leave problems unaddressed or unnoticed.

Exactly. Also mirrors instantly mean you'll pay a 50% premium per GB while RAIDZ2 of 10 disks is only a 20% premium.

 

[Yamanipanuchi]

All very good information, Thank you everyone.

I believe i'm going to work with the group of Raidz2. But i'm noticing with 8-500g drives i am only getting 1.7TB of usable storage. I was expecting a little over 2TB of storage. What am I missing? Im loosing well over %50 I believe.

 

[anodos]

If you have an 8-disk RAIDZ2 zpool you can expect roughly 3TB or 2.7TiB and the ability to sustain up to 2 disk failures before data loss.
You mentioned striping two 4-disk RAIDZ2 vdevs. In this configuration you can expect roughly 2TB or 1.8TiB, and the ability to sustain up to 4 disk failures before data loss.

A terabyte (unit of measurement used by hard drive manufacturers to market products) is 10^12 bytes. A tebibyte (unit of measurement used by computers) is 2^40 bytes. See article here: http://en.wikipedia.org/wiki/Tebibyte

If you need more space go with a single 8-disk RAIDZ2 zpool.

 

[Yamanipanuchi]

Okey, This makes sense. But the numbers im getting seem less then that.

RaidZ2 predicts 2.72TB, But once created I only show 2.5TB.
RaidZ3 predicts 2.26TB, But once created Ionly show 2TB.

RaidZ3 seems to almost be equivalent to a mirror (Space wise).

Does this seem normal, Or am I missing something?

 

[anodos]

This is normal. See here: http://doc.freenas.org/index.php/Hardware_Recommendations#RAID_Overview
RAIDZ2 is double-parity with storage capacity of (roughly) [number of disks -2] * capacity of smallest disk, RAIDZ3 is triple-parity with storage capacity of (roughly) [number of disks -3] * capacity of smallest disk. You have 500GB disks.
In reality you should plan the size of your zpool so that you are using less than 80% of its capacity (otherwise you will start seeing serious performance degradation). So technically you shouldn't go beyond 2.16TB.

 

[joeschmuck]

Use this calculator to give you an idea on what you can expect for a given configuration. Looks like a RAIDZ2 can give you 2.794TB, but then don't forget to subtract the 2GB per drive that gets stolen for swap space, and you now have ~2.778TB.