[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM

[Zdenek Kabelac]

Dne 15.5.2017 v 16:48 Gionatan Danti napsal(a):
> On 15/05/2017 14:50, Zdenek Kabelac wrote> Hi
>>
> What I does not understand is how XFS and EXT4 differs when a thinp is full. 
>  From a previous your reply, after I asked how to put thinp in read only mode 
> when full:
> 
> "Using 'ext4' with remount-ro is fairly easy to setup and get exactly this 
> logic."
> 
> My naive interpretation is that when EXT4 detects *any* I/O error, it will set 
> the filesystem in read-only mode. Except that my tests show that only failed 
> *metadata* update put the filesystem in this state. The bad thingh is that, 
> when not using "remount-ro", even failed metadata updates will *not* trigger 
> any read-only response.


Ever tested this:

mount -o errors=remount-ro,data=journal ?

Everything has it's price - you want to have also 'safe' data - well you have 
to pay the price.


> On the other hand, XFS has not such options but it, by default, ensures that 
> failed *metadata* updates will stop the filesystem. Even reads are not allowed 
> (to regain read access, you need to repair the filesystem or mount it with 
> "ro,norecovery").
> 
> So, it should be even safer than EXT4, right? Or do you feel that is the other 
> way around? If so, why?

I prefer 'remount-ro'  as the FS is still at least accessible/usable in some way.



> 
>> Things are getting better -  but  planning  usage of thin-pool to 'recover' 
>> overfilled pool is simple BAD planning. You should plan your thin-pool usage 
>> to NOT run out-of-space.
> 
> Sure, and I am *not* planning for it. But as bad things always happen, I'm 
> preparing for them ;)

When you have extra space you can add for recovery - it's usually easy.
But you will have much harder time doing recovery without extra space.

So again - all has its price....

Regards

Zdenek