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

Zdenek Kabelac zkabelac at redhat.com
Sat Apr 22 21:17:42 UTC 2017

Dne 22.4.2017 v 18:32 Xen napsal(a):
> Gionatan Danti schreef op 22-04-2017 9:14:
>> Il 14-04-2017 10:24 Zdenek Kabelac ha scritto:
>>> However there are many different solutions for different problems -
>>> and with current script execution - user may build his own solution -
>>> i.e.  call
>>> 'dmsetup remove -f' for running thin volumes - so all instances get
>>> 'error' device   when pool is above some threshold setting (just like
>>> old 'snapshot' invalidation worked) - this way user will just kill
>>> thin volume user task, but will still keep thin-pool usable for easy
>>> maintenance.
>> This is a very good idea - I tried it and it indeed works.
> So a user script can execute dmsetup remove -f on the thin pool?
> Oh no, for all volumes.
> That is awesome, that means a errors=remount-ro mount will cause a remount right?

Well 'remount-ro' will fail but you will not be able to read anything
from volume as well.

So as said - many users many different solutions are needed...

Currently lvm2 can't support that much variety and complexity...

>> However, it is not very clear to me what is the best method to monitor
>> the allocated space and trigger an appropriate user script (I
>> understand that versione > .169 has %checkpoint scripts, but current
>> RHEL 7.3 is on .166).
>> I had the following ideas:
>> 1) monitor the syslog for the "WARNING pool is dd.dd% full" message;
> This is what my script is doing of course. It is a bit ugly and a bit messy by 
> now, but I could still clean it up :p.
> However it does not follow syslog, but checks periodically. You can also 
> follow with -f.
> It does not allow for user specified actions yet.
> In that case it would fulfill the same purpose as > 169 only a bit more poverly.
>> One more thing: from device-mapper docs (and indeed as observerd in my
>> tests), the "pool is dd.dd% full" message is raised one single time:
>> if a message is raised, the pool is emptied and refilled, no new
>> messages are generated. The only method I found to let the system
>> re-generate the message is to deactiveate and reactivate the thin pool
>> itself.
> This is not my experience on LVM 111 from Debian.
> For me new messages are generated when:
> - the pool reaches any threshold again
> - I remove and recreate any thin volume.
> Because my system regenerates snapshots, I now get an email from my script 
> when the pool is > 80%, every day.
> So if I keep the pool above 80%, every day at 0:00 I get an email about it :p. 
> Because syslog gets a new entry for it. This is why I know :p.

The explanation here is simple - when you create a new thinLV - there is 
currently full suspend - and before 'suspend' pool is 'unmonitored'
after resume again monitored - and you get your warning logged again.


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