[linux-lvm] lvm limitations

[Zdenek Kabelac]

Dne 15. 09. 20 v 23:24 Tomas Dalebjörk napsal(a):
> thanks
> 
> ok, lets say that I have 10 LV on a server, and want create a thin lv snapshot every hour and keep that for 30 days
> that would be 24h * 30days * 10lv = 720 lv
> 
> if I want to keep snapshot copies from more nodes, to serve a single repository of snapshot copies, than these would easily become several hundred thousands of lv
> 
> not sure if this is a good idea, but I guess it can be very useful in some sense as block level incremental forever and instant recovery can be implemented for open sourced based applications
> 
> what reflections do you have on this idea?


Hi

You likely don't need such amount of 'snapshots' and you will need to 
implement something to remove snapshot without need, so i.e. after a day you 
will keep maybe 'every-4-hour' and after couple days maybe only a day-level 
snapshot. After a month per-week and so one.

You need to be aware that as soon as your volumes have some 'real-live'
you will soon need to keep many blocks in many 'different' copies which will
surely be reflected by the usage of the pool device itself - aka you will
most likely hit out-of-space sooner then you'll run out of devices.

Speaking of thin volumes - there can be at most 2^24 thin devices
(this is hard limit you've ask for ;)) - but you have only  ~16GiB of metadata 
to store all of them - which gives you ~1KiB of data per such volume -
quite frankly this is not too much  - unless as said - your volumes
are not changed at all - but then why you would be building all this...

That all said -  if you really need that intensive amount of snapshoting,
lvm2 is likely not for you - and you will need to build something on your own,
as you will need way more efficient and 'targeted' solution for your purpose.

There is no practical way to change current LVM2 into a tool handling i.e. 
100.000 LV at decent speed - it's never been a target of this tool....


Regards

Zdenek