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

Zdenek Kabelac zkabelac at redhat.com
Sat Apr 22 21:22:03 UTC 2017

Dne 22.4.2017 v 09:14 Gionatan Danti napsal(a):
> 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.
> 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;
> 2) set a higher than 0  low_water_mark and cache the dmesg/syslog 
> "out-of-data" message;
> 3) register with device mapper to be notified.
> What do you think is the better approach? If trying to register with device 
> mapper, how can I accomplish that?
> 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.

ATM there is even bug for 169 & 170 - dmeventd should generate message
at 80,85,90,95,100 - but it does it only once - will be fixed soon...

>> ~16G so you can't even extend it, simply because it's
>> unsupported to use any bigger size
> Just out of curiosity, in such a case, how to proceed further to regain access 
> to data?
> And now the most burning question ... ;)
> Given that thin-pool is under monitor and never allowed to fill data/metadata 
> space, as do you consider its overall stability vs classical thick LVM?

Not seen metadata error for quite long time...
Since all the updates are CRC32 protected it's quite solid.



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