Striped Mirror vs RAID Z2

[AZ_Pete]

I'm new to FreeNAS and have been doing a lot of reading and research as well as spending quite some time
reading the forums. I feel that I have a good, basic understanding of FreeNas.

My plan is to build a FreeNas box to for home media (movies, music, etc) as well as system backup images,
backups of software code (I'm a software developer) and general file sharing. My plan is to have 4x 3TB hard disks (WD Red), but was a bit confused about the type of pool to build.

My two scenarios are:

A) Two vdevs, each with two 3Tb drives mirrored, then stripe the 2 mirrors. Total Cap. = 6Tb. Drive loss ability = 2 (but only one from each vdev)

B) One vdev, 4x 3Tb drives in RAID Z2. Total Cap. = 6Tb. Drive loss ability = 2

After reading the risks of RAIDZ1, I eliminated that from consideration since data redundancy is very important for me.

Each of the options above provide the same total capacity. From my understanding of RAID configurations, option B has is a bit more resilient in that it can sustain a loss of any two drives without data loss, whereas option A can sustain a loss of two drives, but only if they are in different vdevs. Since both arrangements have the same capacity option B would seem the better choice.

Am I correct in my thinking?

As I understand things option A has better performance, but I was willing to give up some performance for redundancy. Is the performance increase of a striped mirror so significant vs RAIDZ2 that I should go that route? In other words, am I being too overly cautious about losing two drives resulting in giving up significant performance?

I'm hoping I'm not being to "open-ended" with these questions.
Any thoughts are most appreciated.

Thanks,
Peter

 

[gpsguy]

I'll give you an A for your studies. Your thinking is correct.

For your use case, I recommend option B. If you server allows for 6 drives, you might want to consider adding 2 more drives now. It would double your capacity, with the same degree of protection.

 

[joeschmuck]

Is the performance increase of a striped mirror so significant vs RAIDZ2 that I should go that route?

For the configuration you are talking about, it's not going to be significant at all. Also because you are only using it for really for slow applications (streaming video or music and backing up software), the RAIDZ2 will be more than fast enough. And I agree with @gpsguy that option B is a better selection for your needs and heed my warning... Figure in the maximum storage you think you will use over the next 3 years and then double it, purchase storage for that size. Four 3TB drives at RAIDZ2 will net you about 5.5TB of storage, not 6TB. Use a RAID calculator to get an idea of the size and keep in mind that the values are approximate since there is also a small amount of overhead in ZFS.

 

[AZ_Pete]

Thanks guys!

I like the idea you both suggested of adding two extra drives now, yielding an additional 6TB storage (for a total of ~12TB). The case I am considering has 8 drive bays, so adding 2 more drives will be ok. However, doesn't this make expandability a bit more difficult? If I want to add more storage, wouldn't I have a add another 6 drives in a RAIDZ2 configuration, which would be expensive (and need a bigger case)? I had considered how to expand the storage capacity in the future and wasn't quite fond of having add another RAIDZ2 vdev to be striped to the original one.

I had thought that if I kept the original 4 drive RAIDZ2 arrangement and needed more storage at some future time that could I just add another two drives in a mirror arrangement and make a separate pool. I suppose the drawback would be that now I have one pool with ~6Tb (RAIDZ1) and another with the two new drives. Is there some reason not to have multiple pools?

I certainly appreciate your thoughts!

Peter

 

[solarisguy]

You would replace your 4GB drives with 8GB, when you need to upgrade.

 

[gpsguy]

There's nothing wrong with striping additional vdev's to increase the size of your pool, as long as one does it correctly. iXsystems has added seatbelts to FreeNAS in an effort to prevent users from doing it wrong, for example, striping a single disk into a RAIDz2 pool. If that single disk failed - poof, the data was unrecoverable.

So, you could start with 4 disks in RAIDz2 and then stripe in another 4 disk RAIDz2 vdev down the road. The "problem" is, that you'll loose the equivalent of 4 disks to parity. RAIDz2 with 6 disks would yield the same amount of usable space as 2 - 4 disk RAIDz2 vdev's. Or, you might consider 8 disks in RAIDz2.

As @solarisguy said, another way to grow the pool, would be to slowly replace all the drives in a vdev with larger ones. Once all of them had been replaced (and resilvered), the pool would automatically expand, based on the size of the replacement hard disks.

And, as you said, you could create a separate pool containing with one or more mirrored vdev's. With that configuration, you'd need to divide your data and keep track of which was on volume with the mirrors (with less fault tolerance) and which data was on the RAIDz2 volume.

If you had a bigger server, say 12-24+ drives and had a need for both general storage and iSCSI, I'd recommend 2 pools. For general storage, RAIDz2 or z3, and striped mirrors for iSCSI. The latter requires a lot of resources, both in RAM and storage overhead on the mirrored pool.

I had considered how to expand the storage capacity in the future and wasn't quite fond of having add another RAIDZ2 vdev to be striped to the original one.

 

[joeschmuck]

Your question as to expand-ability is why I say that you should make the best guess now and buy what you think you may need for the next 3 years, then double it. Hard drives typically last longer than 3 years but the Reds only have a 3 year warranty so just use that as a starting place. In 3 years time the hard drives will more than likely be larger and cheaper per TB and as @solarisguy stated, you would just replace the current drives with a higher capacity should you need that. As long as you're running RAIDZ2 that will be a safe bet, however I can't stress enough that before doing anything with your pool, backup all your important data first. Better safe than sorry because people do screw up the drive replacement process, we have seen it before and we will see it again.