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

Zdenek Kabelac zkabelac at redhat.com
Tue Sep 12 21:57:01 UTC 2017

Dne 12.9.2017 v 19:14 Gionatan Danti napsal(a):
> On 12/09/2017 16:37, Zdenek Kabelac wrote:
>> ZFS with zpolls with thin with thinpools running directly on top of device.
>> If zpools - are 'equally' fast as thins  - and gives you better protection,
>> and more sane logic the why is still anyone using thins???
>> I'd really love to see some benchmarks....
>> Of course if you slow down speed of thin-pool and add way more 
>> synchronization points and consume 10x more memory :) you can get better 
>> behavior in those exceptional cases which are only hit by unexperienced 
>> users who tends to intentionally use thin-pools in incorrect way.....
> Having benchmarked them, I can reply :)
> ZFS/ZVOLs surely are slower than thinp, full stop.
> However, they are not *massively* slower.

Users interested in thin-provisioning are really mostly interested in 
performance - especially on multicore machines with lots of fast storage with 
high IOPS throughput  (some of them even expect it should be at least as good 
as linear....)

So ATM it's preferred to have more complex 'corner-case' which really mostly 
never happens when thin-pool is operated properly and the remaining use case 
you don't pay higher price for having all data always in sync and also you get 
way lower memory foot-print
(I think especially ZFS is well known for nontrivial memory resource consumption)

As has been pointed already few times in this thread - lots of those
'reserved space' ideas can be already easily handled by just more advanced 
scripting around notification from dmeventd - if you will keep thinking for a 
while you will at some point see the reasoning.

There is no difference if you start to solve problem around 70% fullness then
100%  - the main difference is - with some 'free-space' in thin-pool you can 
resolve problem way more easily and correctly.

Repeated again - whoever targets for 100% full thin-pool usage has 
misunderstood purpose of thin-provisioning.....



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