Re: repeated enospc errors during balance on a filesystem with spare room - pls advise

[Ole Langbehn <neurolabs.de@gmail.com>]

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Thanks for the insights (also in other responses).

On 31.12.19 23:58, Zygo Blaxell wrote:
> Two workarounds:
> 
> 	- use 5.3.18 instead of 5.4.6
> 
> 	- use the 'metadata_ratio=1' mount option after balancing a few
> 	data block groups

Switched back to 5.3.18. Since the space bug (at least the external/df
part) was solved for me before switching, I did not need to do anything
further. A full(100%) data balance succeeds successfully now.

> In your other mail you indicated you were running a full balance.  Full
> balances are never useful(*) and will make this specific situation worse.
> 
> A full balance includes a metadata balance.  The primary effect
> of metadata balance is to temporarily reduce space for metadata.
> Reducing metadata space causes an assortment of problems for btrfs,
> only one of which is hitting the 5.4 free space bug.  For all but a few
> contrived test cases, btrfs manages metadata well without interference
> from balance.  Too much metadata balancing (i.e. any at all) can make
> a filesystem truly run out of metadata space on disk--a condition that
> is sometimes difficult to reverse.

In my situation, my observation was that when I hit the space bug, I had
12GB meta reserved, 10.x used. The first balance lifted reserved space
up to 13GB and I have not seen the issue afterwards. So from practice/my
observations, it fixed the issue (also see Swamis response). I will take
into account your insights in the future, though, and try to not balance
metadata.

Cheers,

Ole


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