[DRBD-user] Regarding Best way for snapshotting with ZFS/LVM as backing disk for drbd

[Michael Labriola]

On Mon, Aug 1, 2022 at 4:38 AM Chitvan Chhabra <chitvan1988 at gmail.com> wrote:
>
> All Suggestions are most welcome.
>
> On Fri, 29 Jul 2022 at 16:58, Chitvan Chhabra <chitvan1988 at gmail.com> wrote:
>>
>> Please suggest the Best Way to snapshot Secondary DRBD Volume(Upper) on LVM/ZFS(Backing) using protocol A i.e Minor Data Loss(few Seconds) is OK but data should remain crash consistent like Database.Also Metadata is external in secondary. So Do i need to snapshot external metadata as well. if yes Why ?
>>
>>
>> On Above few Queries are detailed below:
>>
>>
>> 1)As per valuable suggestions by community that normal ZFS/LVM Snapshots would do as long as it is upto date? How to know that in realtime uptodate Status as drdbadm status command doesnot appear to be realtime in nature as it takes time to reflect actual Status for example when primary goes down , only after few seconds only it shows as connecting

I've used ZFS volumes to back my DRBD resources for years, relying on
zfs-auto-snapshot and the Copy On Write nature of ZFS to keep things
recoverable in case of some sort of corruption or human error.

I'm not sure how to answer your "realtime uptodate" question.  I've
never run into data loss corner cases with nodes going down,
reconnecting, etc.

>>
>>
>> 2) Community has again suggested for internal only metadata to have simultaneous/atomic snapshot of both data and metadata .If the primary is having external metadata , it makes sense to have external metadata in secondary as well. But Why do we need to preserve the metadata via snapshot at the first place as it is believed that once you rollback the using the snapshot drbd would get confused and would attempt resynchronisation of the entire device again any way
>>

I've never played with external metadata, so don't have any real world
experience to answer this one.  Internal aways seems like a much
better solution to me.

>>
>> 3) Do i need to suspend-io first before taking the snapshot and then check up to date Status mandatorily ?

I guess that depends on your volume management.  If ZFS volumes, I'm
going to say no that's not necessary, at least I've never gotten
burned by not doing a suspend-io.  LVM, I'm not sure.

>>
>>
>> 4) Last Query is regarding metadata, Does Metadata contains actual data in the form of Activity log/extents.Then what is drbdadm apply-al is for?Depending on the reply of this query we would be able to get crash consistent data (via backing disk snapshot clone ofcourse) even if we loose the metadata
>>

No idea.  Someone else will have to chime in to answer that one.

>>
>> Thanks and Regards,
>>
>> Chitvan Chhabra
>>
>> +919911208768
>
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