On 2018-01-10 16:37, waxhead wrote:
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.
In an ideal situation, the only reason it should ever be a bad idea to
run a balance is the performance impact (which is of course why we have
filters). Beyond that though, there's too much involved for even a
computer to reliably tell you if it will be beneficial to run a balance
or not. It depends not just on how the data looks on the filesystem,
but also how you are going to be using the filesystem in the near future
(for example, if you've got a number of large blocks of empty space
within data chunks, it might make sense to balance, but not if you're
likely to be adding a bunch of new files in the very near future (they
will just end up packed into that empty space in existing chunks, and
your actual layout on disk shouldn't be all that different from if you
had run a balance)).
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 this case, the recommendation isn't as much about speed as it is
about trying to keep things from getting into a state where you get
ENOSPC but conventional tools report lots of free space. As a general
rule, unless things are pathologically bad to begin with, balancing a
filesystem won't usually have any measurable impact on performance.
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 :)
Long-term, that type of things is a goal, but I doubt that we're going
to go that far with automation (even ZFS doesn't go that far, you still
have to schedule scrubs and similar things).
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