Volume expansion failure tolerance

[CompEngLSUFan]

Hello all,

I've been searching google and these forums for several days now and I can't find a straight answer to my question. I have a very simple FreeNas server setup with 2, 3TB WD Red drives configured in a mirror (no dedup or encryption). I have 2 more identical 3TB WD Red drives that I would like to add to this volume. I've read over and over that once a vdev is created, it can't be modified but that you can add more vdevs to a zvol for expansion. I've found the "Volume to extend" drop down box in the volume manager but I can't seem to find a straight answer to the question, if I add two more drives to a volume and any one of the 4 drives, be it one of the original or one new, fails, will the volume still be functional and, with data intact, be in a state where I can replace the failed drive? It seems logical that it would but I've had plenty of experience where what was logical wasn't reality and I don't have any spare drives that I can use to test with. I've also read some information that only new data will be written to all 4 drives of this expanded volume, which makes no sense, but again, I couldn't find any proof against it. Clarification will be much appreciated.

Build: FreeNAS-9.3-STABLE-201512121950
Platform: AMD Phenom(tm) II X4 920 Processor
Memory: 4GB
Disk 1: WD Red 3TB
Disk 2: WD Red 3TB

 

[BigDave]

The loss of a Vdev, loses your whole pool. So you are thinking correctly.
As far as new data is concerned after adding the second Vdev which is
empty at the beginning AND depending how full your original pool was
before expansion, you could be dealing with a bit of imbalance which
might have a slight effect on performance.
If your OCD is kicking in, you could offload all your files, do the expansion
and then load the files back on

 

[CompEngLSUFan]

Thank you, BigDave. So it seems like if a drive in a vdev is still healthy then the data will be safe. My current vdev is about half full so the balance will probably be pretty skewed. Given this information, I think what I'll probably do is copy the data to a temp location and create a RAIDZ2 out of my 4, 3TB drives and copy the data back, unless someone has a better suggestion, which I'm open to.

 

[jde]

Raidz2 will give you better protection from disk failure in that it will allow you to lose any two drives without losing your pool.

Just a note: your system has half the minimum memory, and non-ECC memory at that. This can can cause instability and potentially data corruption. https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-ram-and-zfs.15449/

 

[CompEngLSUFan]

Thanks, jde. That's probably what I'll do is just create a new pool with RAIDz2. I don't need space as much as I need to protect the data from loss. 6TB should last me a good long time.

I know that I don't have good specs on this particular box but it was just a box I had laying around so I could replace some standalone NAS boxes that were too slow. I am currently gathering parts to replace this machine and it will have at least 16GB of RAM, still not ECC though because this is a personal system and I don't use dedup or encryption.

 

[Mirfster]

still not ECC though because this is a personal system and I don't use dedup or encryption.

Just note what jde is trying to warn you about really has nothing to do with dedup or encryption.

 

[CompEngLSUFan]

Yeah, after I posted my reply I started reading the link he supplied. Kind of getting off topic for this thread but I already purchased parts to build another FreeNas server which is also in use right now. I'm not sure how I missed the recommendation/requirement for ECC ram when I was compiling parts for my build but now, I'm scared and don't have budget to rebuild that machine.

 

[Mirfster]

Maybe you could contact the vendor and return the items you just purchased... Or possibly sell them to a close friend or on eBay? I dunno, just some suggestions.

 

[BigDave]

Yeah, after I posted my reply I started reading the link he supplied. Kind of getting off topic for this thread but I already purchased parts to build another FreeNas server which is also in use right now. I'm not sure how I missed the recommendation/requirement for ECC ram when I was compiling parts for my build but now, I'm scared and don't have budget to rebuild that machine.

A lot of the members in here have links in their signtures to some of the forum content they feel are important
information so you can enjoy the use of FreeNAS and it's safeguards for your data. @Mirfster has what most
of us consider the Top Four most important reads for the FreeNAS noob.
You must read and comprehend these threads to become a FreeNAS Jedi ;)