Re: Recommendations for balancing as part of regular maintenance?

[waxhead <waxhead@dirtcellar.net>]

Austin S. Hemmelgarn wrote:
> So, for a while now I've been recommending small filtered balances to
> people as part of regular maintenance for BTRFS filesystems under the
> logic that it does help in some cases and can't really hurt (and if done
> right, is really inexpensive in terms of resources).  This ended up
> integrated partially in the info text next to the BTRFS charts on
> netdata's dashboard, and someone has now pointed out (correctly I might
> add) that this is at odds with the BTRFS FAQ entry on balances.
>
> For reference, here's the bit about it in netdata:
>
> You can keep your volume healthy by running the `btrfs balance` command
> on it regularly (check `man btrfs-balance` for more info).
>
>
> And here's the FAQ entry:
>
> Q: Do I need to run a balance regularly?
>
> A: In general usage, no. A full unfiltered balance typically takes a
> long time, and will rewrite huge amounts of data unnecessarily. You may
> wish to run a balance on metadata only (see Balance_Filters) if you find
> you have very large amounts of metadata space allocated but unused, but
> this should be a last resort.
>
>
> I've commented in the issue in netdata's issue tracker that I feel that
> the FAQ entry could be better worded (strictly speaking, you don't
> _need_ to run balances regularly, but it's usually a good idea). Looking
> at both though, I think they could probably both be improved, but I
> would like to get some input here on what people actually think the best
> current practices are regarding this (and ideally why they feel that
> way) before I go and change anything.
>
> So, on that note, how does anybody else out there feel about this?  Is
> balancing regularly with filters restricting things to small numbers of
> mostly empty chunks a good thing for regular maintenance or not?
> --
As just a regular user I would think that the first thing you would need 
is an analyze that can tell you if it is a good idea to balance or not 
in the first place.

Scrub seems like a great place to start - e.g. scrub could auto-analyze 
and report back need to balance. I also think that scrub should 
optionally autobalance if needed.

Balance may not be needed, but if one can determine that balancing would 
speed up things a bit I don't see why this as an option can't be 
scheduled automatically. Ideally there should be a "scrub and polish" 
option that would scrub, balance and perhaps even defragment in one go.

In fact, the way I see it btrfs should idealy by itself keep track on 
each data/metadata chunk and it should know , when was this chunk last 
affected by a scrub, balance, defrag etc and perform the required 
operations by itself based on a configuration or similar. Some may 
disagree for good reasons , but for me this is my wishlist for a 
filesystem :) e.g. a pool that just works and only annoys you with the 
need of replacing a bad disk every now and then :)

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