You will want to read about aggr re-allocate, vol realloate. There was a GREAT blog out there somwhere with cool pictures, and levels of data as they are laid down, so you can start off with a RG of 16, start with 6, and add (at min) 3 disks at a time. Run the re-allocate to rebalance, and you are there..
I think thats how it is... Bottom line, its best to add as many as you can.. repeat, rinse.. Repeat if still dirty..
> Subject: from performance vs rg size to new topic: rg size uniformity within the aggregate
> Date: Tue, 1 Nov 2011 19:53:59 -0500
> From: recotton@uif.uillinois.edu
> To: toasters@teaparty.net
>
> Unfortunately, I've not been enlightened as to performance numbers for
> varying small aggregate sizes with one raid group, so I guess I'll have
> to be content with shooting in the dark by starting out small and
> growing from there if the performance is too lousy. Regardless, may
> comments were still helpful and they are much appreciated.
>
> But now to a tangentially related subject:
>
> >From recent posts, one might be led to believe that it really doesn't
> matter much at all what the sizes of your raid groups are within an
> aggregate. One might get the idea that an aggregate with, say, 3 raid
> groups of size 6, 12 and 18 wouldn't perform much differently than one
> with 3 identical raid groups of size 12 because what matters far more
> than anything else is the data disk count of an aggregate:
>
> For instance:
>
> "...there is no reason one 4+2 and two 16+2s cant perform quite well.
> There is no unreasonable penalty to doing so ... if the right tools are
> used to manage it."
>
> "Performance of an aggregate is the total sum of spindles across the
> entire aggregate."
>
> And
>
> "Forget parity as a limitation to system performance"
>
> However, NetApp's recommendations on raid group sizing within the
> aggregate might lead one to believe somewhat the opposite:
>
> >From http://communities.netapp.com/thread/1587 :
>
> "the recommended sizing approach is to establish a RAID group size that
> ... achieves an even RAID group layout (all RAID groups contain the same
> number of drives). If multiple RAID group sizes achieve an even RAID
> group layout, NetApp recommends using the higher RAID group size value
> within the range."
>
> and
>
> "Drive deficiencies should be distributed across RAID groups so that no
> single RAID group is deficient more than a single drive."
>
> That is, NetApp took the time and trouble to say:
>
> 1. RAID group sizes within an aggregate shouldn't vary by more than 1
> disk
> 2. Larger RAID group sizes are better (up to a point)
>
> Which runs rather strongly against the grain of what's been said
> recently in this forum. Can anyone shed more light on this topic?
>
> For instance, what are the drawbacks (and how bad are they) to, say,
> adding a 6-disk raid group to an aggregate consisting of one 16-disk
> raid group (instead of spilling for way more disk space than you might
> need by forking over for the full 16 disks necessary to even things
> out?).
>
> Thanks
> Randall
>
>
>
> _______________________________________________
> Toasters mailing list
> Toasters@teaparty.net
> http://www.teaparty.net/mailman/listinfo/toasters