28 May
2020
28 May
'20
1:41 p.m.
On Thu, May 28, 2020 at 8:44 AM Hans van den Bogert <hansbogert(a)gmail.com>
wrote:
Hi list,
When reading the documentation for the new way of mirroring [1], some
questions arose, especially with the following sentence:
I agree that it's probably a poor choice of verbiage. Both are technically
crash / point-in-time consistent. The difference is that journal-based
mirroring provides a replayable stream of updates that are synced with a
very fine granularity (down to single IO updates) whereas snapshot-based
mirroring has a very coarse granularity.
2) Also, I am not sure what the implications are for the second part of
Since this mode is not point-in-time consistent,
the full snapshot
delta will need to be synced prior to use during a failover
scenario.
1) I'm not sure I follow. Why are snapshots not point-in-time, whereas
using the journal-based mirroring is?
the referenced sentence, what significance does the
word "full" bear in
"full snapshot delta" ?
Given mirroring snapshot A and B, the full delta of changes between
snapshots A and B will need to be successfully synced / applied to the
non-primary image. If you force a failover before a the delta between
snapshot A and B can be applied, the system will rollback the non-primary
image to snapshot A to get the non-primary image to a consistent state.
Further, Isn't it obvious that in order to do a
failover, the other side should have the latest snapshot? - I am
probably missing what the documentation is trying to tell me.
Not all failovers are planned.
All in all, I think some background concepts are missing in order to
read this chapter of the documentation. It might well
be that this is
explained somewhere else in the docs, but maybe we should link to that.
If someone can explain the above 2 points, I'm willing to take a shot at
making a patch if I can make this clearer in the docs.
Please feel free to submit any clarifying updates.
Thanks,
Hans
[1] https://docs.ceph.com/docs/octopus/rbd/rbdcourse-mirroring/
<https://docs.ceph.com/docs/octopus/rbd/rbd-mirroring/>
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--
Jason