[linux-lvm] Reserve space for specific thin logical volumes

[Gionatan Danti]

Il 13-09-2017 10:15 Zdenek Kabelac ha scritto:
> Ohh this is pretty major constrain ;)

Sure :p
Sorry for not explicitly stating that before.

> 
> But as pointed out multiple times - with scripting around various
> fullness moments of thin-pool - several different actions can be
> programmed around,
> starting from fstrim, ending with plain erase of unneeded snapshot.
> (Maybe erasing unneeded files....)
> 
> To get most secure application - such app should actually avoid using
> page-cache (using direct-io)  in such case you are always guaranteed
> to get exact error at the exact time (i.e. even without journaled
> mounting option for ext4....)
> 
> 

True, but pagecache exists for a reason. Anyway, this is not anything 
you can "fix" in device mapper/lvm, I 100% agree with that.

> Partially this might get solved in 'some' cases with fully provisioned
> thinLVs within thin-pool...
> 
> What comes to my mind as possible supporting solution is -
> adding possible enhancement on LVM2 side could be  'forcible' removal
> of running volumes  (aka lvm2 equivalent  of 'dmsetup remove --force')
> 
> ATM lvm2 prevents you to remove 'running/mounted' volumes.
> 
> I can well imagine  LVM will let you forcible  replace such LV with
> error target  - so instead of  thinLV  - you will have  single 'error'
> target snapshot - which could be possibly even  auto-cleaned once the
> volume use-count drops bellow 0  (lvmpolld/dmeventd monitoring
> whatever...)
> 
> (Of course - we are not solving what happens to application
> using/running out of such error target - hopefully something not
> completely bad....)
> 
> This way - you get very 'powerful' weapon to be used in those 
> 'scriplets'
> so you can drop uneeded volumes ANYTIME you need to and reclaim its 
> resources...
> 

This would be *really* great. I played with dm-setup remove/error target 
and, while working, it often freezed LVM.
An integrated forced volume removal/swith to error target would be 
great.

Thank.

-- 
Danti Gionatan
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