Re: Ideas to do custom operation just after mount?

[Duncan <1i5t5.duncan@cox.net>]

Kai Krakow posted on Tue, 22 Dec 2015 01:14:24 +0100 as excerpted:

> Am Mon, 21 Dec 2015 13:18:22 +0800 schrieb Anand Jain
> <anand.jain@oracle.com>:
> 
> 
>> 
>> > BTW, any good idea for btrfs to do such operation like
>> > enabling/disabling some minor features? Especially when it can be set
>> > on individual file/dirs.
>> >
>> > Features like incoming write time deduplication, is designed to be
>> > enabled/disabled for individual file/dirs, so it's not a quite good
>> > idea to use mount option to do it.
>> >
>> > Although some feature, like btrfs quota(qgroup), should be
>> > implemented by mount option though.
>> > I don't understand why qgroup is enabled/disabled by ioctl. :(
>> 
>> 
>> mount option won't persist across systems/computers unless remembered
>> by human.
> 
> Valid point, tho here's an counter-example: space-cache is persisted.
> Once enabled, it's always there until you clear AND disable it during
> initial mount.

Data point.  space_cache doesn't have to be manually enabled at all, 
unless it was turned off at some point; it's automatically enabled on 
mounting new btrfs.

At least, I've never specifically enabled (or disabled) space_cache on 
any of my btrfs here, yet they all have it enabled.

(Years ago, with my first btrfs experiments, a corrupted space_cache 
could cause the filesystem not to mount.  But then I left btrfs alone for 
awhile, and I guess in the mean time it learned to auto-correct such 
problems, as I've never had to worry about a bad space-cache failing a 
mount since I returned to using it, and that itself was two and a half 
years ago, now.  With the previous mount-blocker experience I was a 
rather nervous when I saw it enabled without me initially enabling it, 
but space_cache has basically "just worked" for me since then, tho I've 
had a few other mount-blocking bugs.)

-- 
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"Every nonfree program has a lord, a master --
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