Expanding a pool vs larger pool now

[designflaw]

Hi All - first I want to say thanks for all the helpful posts/guides/discussions on here. You are an awesome resource for a FreeNAS noob. I'm currently planning on building a box with the following hardware:

ASRock C2750D4I
2x8GB Crucial ECC ram
Fractal Node 804 Case
450W PSU

And for drives... I'm stuck. I'm planning on at least doing 6x3TB WD Reds at $110 each configured in RAIDZ2. I think I'll hit capacity in about 18 months and will need to expand my pool. I read in cyberjocks slideshow that it is recommended to add a new vdev with the same number of drives but I'm not clear on why. What would the impact be if I expanded my pool by adding a new RAIDZ2 vdev made up of 4 disks in this situation? If it's significant I'm thinking I'll need to reevaluate the case I've decided on as it only supports 10 drives. I'm also considering just doing 10 drives initially to avoid any potential issues.

TL;DR - Is it ok to add a 4 disk vdev to a pool with a 6 disk vdev or would it be better to start out with a 10 disk vdev? (All vdevs being RAIDZ2)

Thanks!

 

[enemy85]

To expand your pool in the future, you can even just substitute the 3TB disks with bigger ones. Concerning your question, i don't know if there would be any performance issue, but the most important thing in adding new vdevs to an existing pool, is just keeping the same redundance level (raidz2 with raidz2 etc)

 

[jgreco]

most important thing in adding new vdevs to an existing pool, is just keeping the same redundance level (raidz2 with raidz2 etc)

I'm not clear on why you think that'd be. It's a good idea, yes, but mostly to avoid confusion. You can absolutely mix and match vdevs. However, your pool will only be as strong as the weakest vdev. That probably also explains

it is recommended to add a new vdev with the same number of drives but I'm not clear on why.

Because the new vdev will perform similarly to the old vdev that way, and ZFS will attempt to balance I/O between the two pools, so if one is way slow and the other is way fast, you end up losing lots of speed because the slow vdev is messing with your throughput.

TL;DR - Is it ok to add a 4 disk vdev to a pool with a 6 disk vdev or would it be better to start out with a 10 disk vdev? (All vdevs being RAIDZ2)

Thanks!

Yes it's fine to do that but you Really Don't Want To. Economically it makes no sense. Get your ten 3TB disks in RAIDZ2, that gives you 24TB usable for $1100. The 6 disk vdev would give you 12TB plus the 4 disk vdev would give you 6TB for a total of 18TB for the same price.

 

[designflaw]

Yes it's fine to do that but you Really Don't Want To. Economically it makes no sense. Get your ten 3TB disks in RAIDZ2, that gives you 24TB usable for $1100. The 6 disk vdev would give you 12TB plus the 4 disk vdev would give you 6TB for a total of 18TB for the same price.

Wow thanks - I'm kicking myself for not realizing that. Clearly I should do a single 10 disk vdev in this case. I'm realizing that the bulk of my data will be backed up so it is an option to just blow away the vdev and create a new one with the additional drives when I need to. The question I now have to answer myself is.. is it worth the $440 now to save some hassle later. I'll have to think on that. :)

Another thought.. Since the data can be restored from backup would I be ok with doing RAIDZ1 and 2 separate 5 disk vdevs? I know there is an increased risk when rebuilding with single parity but if the data can be restored is that risk typically considered acceptable?

 

[cyberjock]

Another thought.. Since the data can be restored from backup would I be ok with doing RAIDZ1 and 2 separate 5 disk vdevs? I know there is an increased risk when rebuilding with single parity but if the data can be restored is that risk typically considered acceptable?

That depends. How much inconvenience are you willing to deal with when the rebuild fails?

 

[designflaw]

I suppose thats a question I need to answer myself - not sure why I asked :) Right now I'm leaning toward just starting with 10 and calling it a day. A lot of this space will be unused for awhile but at least it's there and there will be no monkey business when I do need it. Thanks for the input.

 

[enemy85]

" However, your pool will only be as strong as the weakest vdev. That probably also explains
.

That was the suggestion i was referring to

 

[designflaw]

I just found this thread - https://forums.freenas.org/index.php?threads/number-of-disks-in-vdev.16956/

It would be closer to my budget if I went with 8 disks now - I have largely the same usecase - storing media and serving it via shares and plex.

Tho 8 isn't optimal for RAIDZ2 would it be ok in this case? I suppose I could just build it and test file transfer speed. If it looks bad I can just pick up 2 additional drives.

 

[cyberjock]

Your question is answered in my noobie presentation. ;)

 

[designflaw]

Ah.. so it is. Thanks.. I will disregard that thumbrule for now. (I swear I've read thru that a few times - there's just so much useful info there it's hard to remember it all :p)