On 2018-01-09 03:33, Marat Khalili wrote:
On 08/01/18 19:34, Austin S. Hemmelgarn wrote:
A: While not strictly necessary, running regular filtered balances
(for example `btrfs balance start -dusage=50 -dlimit=2 -musage=50
-mlimit=4`, see `man btrfs-balance` for more info on what the options
mean) can help keep a volume healthy by mitigating the things that
typically cause ENOSPC errors.
The choice of words is not very fortunate IMO. In my view volume
stopping being "healthy" during normal operation presumes some bugs (at
least shortcomings) in the filesystem code. In this case I'd prefer to
have detailed understanding of the situation before copy-pasting
commands from wiki pages. Remember, most users don't run cutting-edge
kernels and tools, preferring LTS distribution releases instead, so one
size might not fit all.
I will not dispute that the tendency of BTRFS to end up in bad
situations is a shortcoming of the filesystem code. However, that isn't
likely to change any time soon (fixing it is going to be a lot of work
that will likely reduce performance for quite a few people), so there is
absolutely no reason that people should not be trying to mitigate the
problem.
As far as the exact command, the one I quoted has worked for at least 2
years worth of btrfs-progs and kernels, and I think far longer than that
(the usage and limit filters were implemented pretty early on). I agree
that detailed knowledge would be better, but that doesn't exactly fit
with the concept of a FAQ in most cases, and most people really don't
care about the details as long as it works.
On 08/01/18 23:29, Martin Raiber wrote:
There have been reports of (rare) corruption caused by balance (won't be
detected by a scrub) here on the mailing list. So I would stay a away
from btrfs balance unless it is absolutely needed (ENOSPC), and while it
is run I would try not to do anything else wrt. to writes simultaneously.
This is my opinion too as a normal user, based upon reading this list
and own attempts to recover from ENOSPC. I'd rather re-create filesystem
from scratch, or at least make full verified backup before attempting to
fix problems with balance.
While I'm generally of the same opinion (and I have a feeling most other
people who have been server admins are too), it's not a very user
friendly position to recommend that. Keep in mind that many (probably
most) users don't keep proper backups, and just targeting 'sensible'
people as your primary audience is a bad idea. It also needs to work at
at least a basic level anyway though simply because you can't always
just nuke the volume and rebuild it from scratch.
Personally though, I don't think I've ever seen issues with balance
corrupting data, and I don't recall seeing complaints about it either
(though I would love to see some links that prove me wrong).
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