Gionatan Danti schreef op 18-09-2017 21:20:
Xen, I really think that the combination of hard-threshold obtained by
setting thin_pool_autoextend_threshold and thin_command hook for
user-defined script should be sufficient to prevent and/or react to
full thin pools.
I will hopefully respond to Zdenek's message later (and the one before
that that I haven't responded to),
I'm all for the "keep it simple" on the kernel side.
But I don't mind if you focus on this,
That said, I would like to see some pre-defined scripts to easily
manage pool fullness. (...) but I would really
like the standardisation such predefined scripts imply.
And only provide scripts instead of kernel features.
Again, the reason I am also focussing on the kernel is because:
a) I am not convinced it cannot be done in the kernel
b) A kernel feature would make space reservation very 'standardized'.
Now I'm not convinced I really do want a kernel feature but saying it
isn't possible I think is false.
The point is that kernel features make it much easier to standardize and
to put some space reservation metric in userland code (it becomes a
default feature) and scripts remain a little bit off to the side.
However if we *can* standardize on some tag or way of _reserving_ this
space, I'm all for it.
I think a 'critical' tag in combination with the standard
autoextend_threshold (or something similar) is too loose and ill-defined
and not very meaningful.
In other words you would be abusing one feature for another purpose.
So I do propose a way to tag volumes with a space reservation (turning
them cricical) or alternatively to configure a percentage of reserved
space and then merely tag some volumes as critical volumes.
I just want these scripts to be such that you don't really need to
modify them.
In other words: values configured elsewhere.
If you think that should be the thin_pool_autoextend_threshold, fine,
but I really think it should be configured elsewhere (because you are
not using it for autoextending in this case).
thin_command is run every 5%:
https://www.mankier.com/8/dmeventd
You will need to configure a value to check against.
This is either going to be a single, manually configured, fixed value
(in % or extents)
Or it can be calculated based on reserved space of individual volumes.
So if you are going to have a kind of "fsfreeze" script based on
critical volumes vs. non-critical volumes I'm just saying it would be
preferable to set the threshold at which to take action in another way
than by using the autoextend_threshold for that.
And I would prefer to set individual space reservation for each volume
even if it can only be compared to 5% threshold values.
So again: if you want to focus on scripts, fine.
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