Re: [PATCH v5 18/19] btrfs-progs: mkfs: create the global root's

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Mar 09, 2022 at 07:35:34PM +0100, David Sterba wrote:
> On Mon, Mar 07, 2022 at 05:11:03PM -0500, Josef Bacik wrote:
> > Now that we have all of the supporting code, add the ability to create
> > all of the global roots for an extent tree v2 fs.  This will default to
> > nr_cpu's, but also allow the user to specify how many global roots they
> > would like.
> 
> Why is number of online cpus a good default? Or how a user should know
> what's a good number? It resembles the allocation groups on xfs that
> are set at mkfs time and once the filesystem is grown the size remains
> but the number explodes and becomes problematic if the the old/new sizes
> are disproportionate. We have more flexibility in btrfs with the resize
> so we could afford to set the intial number based rather on the device
> size and then a rebalance after resize can adjust that again. Maybe
> there's something in kernel taking care of that, I don't know.

Right now I have no idea what a good number is, so I'm defaulting to NR_CPUS.  I
*think* this is a good idea because generally we want to spread the locking
pain, so hopefully NR_CPU's is good enough for most people?

We allow setting your own number if a user does the benchmarking to find their
ideal global roots number (*cough*Zygo*cough*).  And it'll be easy enough to add
new global roots since we tie the block group to the global root at create time.
Thanks,

Josef