[linux-lvm] Reserve space for specific thin logical volumes
g.danti at assyoma.it
Tue Sep 12 23:15:06 UTC 2017
> There could be a simple answer and complex one :)
> I'd start with simple one - already presented here -
> when you write to INDIVIDUAL thin volume target - respective dn thin
> target DOES manipulate with single btree set - it does NOT care there
> are some other snapshot and never influnces them -
> You ask here to heavily 'change' thin-pool logic - so writing to THIN
> volume A can remove/influence volume B - this is very problematic for
> meny reasons.
> We can go into details of BTree updates (that should be really
> discussed with its authors on dm channel ;)) - but I think the key
> element is capturing the idea the usage of thinLV A does not change
> thinLV B.
> Now to your free 'reserved' space fiction :)
> There is NO way to decide WHO deserves to use the reserve :)
> Every thin volume is equal - (the fact we call some thin LV snapshot
> is user-land fiction - in kernel all thinLV are just equal - every
> thinLV reference set of thin-pool chunks) -
> (for late-night thinking - what would be snapshot of snapshot which
> is fully overwritten ;))
> So when you now see that all thinLVs just maps set of chunks,
> and all thinLVs can be active and running concurrently - how do you
> want to use reserves in thin-pool :) ?
> When do you decide it ? (you need to see this is total race-lend)
> How do you actually orchestrate locking around this single point of
> failure ;) ?
> You will surely come with and idea of having reserve separate for every
> thinLV ?
> How big it should actually be ?
> Are you going to 'refill' those reserves when thin-pool gets emptier ?
> How you decide which thinLV deserves bigger reserves ;) ??
> I assume you can start to SEE the whole point of this misery....
> So instead - you can start with normal thin-pool - keep it simple in
> and solve complexity in user-space.
> There you can decide - if you want to extend thin-pool...
> You may drop some snapshot...
> You may fstrim mounted thinLVs...
> You can kill volumes way before the situation becomes unmaintable....
Ok, this is an answer I totally accept: if enable per-lv used and
reserved space is so difficult in the current thinp framework, don't do
Thanks to taking the time to explain (on late night ;))
> All you need to accept is - you will kill them at 95% -
> in your world with reserves it would be already reported as 100% full,
> with totally unknown size of reserves :)
Minor nitpicking: I am not speaking about "reserves" to use when free
space is low, but about "reserved space" - ie: per-volume space which
can not be used by any other object.
One question: in a previous email you shown how a threshold can be set
to deny new volume/snapshot creation. How can I do that? What LVM
version I need?
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email: g.danti at assyoma.it - info at assyoma.it
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