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

[Gionatan Danti <g.danti@assyoma.it>]

> 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 
> kernel,
> 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 
it.

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?

Thanks.

-- 
Danti Gionatan
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