On 2016-08-25 05:38, Holger Hoffstätte wrote:
Automatically resuming an interrupted balance has repeatedly caused all
sorts of problems because it creates a possible failure mode when a user
can least use it: after a crash/power loss/sudden reboot (which, like it
or not, is the de facto "fix random problems" approach for many people).
The idea behind the automnatic resume is good and important for cases
like automation and unattended operation, but nevertheless right now
it creates more problems than it fixes.
I would tend to agree.
If the system crashed in the middle of a balance, then we can't assume
it was something other than the filesystem being balanced that caused
the crash. Resuming a balance in such a situation is risky and could
even trigger another crash (yes, I've actually seen this happen, and it
was a serious pain to debug). If the balance was paused so the system
could reboot with the intent of resuming it later, then the user knows
they did this, and should be more than capable of resuming it themself.
I've gotten to the point that all of the filesystems I manage have
'skip_balance' in their mount options to avoid all the issues I've had,
and because resuming a balance on a filesystem which is mounted on boot
can insanely slow down the boot process.
As far as I can see it should be easy enough to simply disable calling
btrfs_resume_balance_async() at least on mount (in open_ctree()) and
possibly on remount as well. The skip_balance flag could then simply
be ignored or removed.
I can't say how much work it would be to completely remove the persistent
balance state or whether it is useful to be kept around for resume, but at
least not continuing would stop filesystems from eating themselves further
on mount.
I think we should keep the persistent balance state, it has on multiple
occasions been useful to me (running a balance, power loss or some other
condition requiring shutdown imminent, so I paused, shutdown, and then
resumed after reboot), and removing it completely may break some things.
Personally, I think we should add an option to force resuming a balance
on mount (resume_balance maybe?), and then switch to skip_balance being
default. This would allow people who still want the auto-resume
behavior to keep it, while making the defaults safer.
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