Re: Recommendations for balancing as part of regular maintenance?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

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).