Re: [bcachefs] time of mounting filesystem with high number of dirs

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Sep 07, 2016 at 10:09:58PM +0200, Marcin wrote:
> Hi!
> I'm aware that performance doesn't have high priority, it's something for
> TODO.
> I created bcachefs on ~10GB partition, copied some files and next I created
> huge number of directories and files. Problem is in the total time needed
> for mounting filesystem.
> # time mount -t bcache /dev/sde1 /mnt/test/
> 
> real    12m20.880s
> user    0m0.000s
> sys     1m18.270s

Oh damn, guess it's time to start working on mount time... I knew this was going
to be an issue sooner or later, but 12 minutes is impressive :)

> I looked at iostat, mounting needs to read from disk 10083588 "kB_read".
> Device has size 10485760kB, so it looks that it reads almost the same amount
> of data as partition size. Maybe mount time can be lower? Maybe there can be
> less reads or reads could be more sequential?

So, right now we're checking i_nlinks on every mount - mainly the dirents
implementation predates the transactional machinery we have now. That's almost
definitely what's taking so long, but I'll send you a patch to confirm later.

It shouldn't take that much work to make the relevant filesystem code
transactional, I'll bump that up on the todo list...