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Stefan Miklosovic commented on CASSANDRA-16911:
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One not so obvious consequence of this patch is that, until now, there was a
"scrubbing" process as part of startup checks which scanned the directories and
removed these ephemeral snapshots when that marker file was found.
Since we are using SnapshotManager from now on (or at least we try to drift
towards that), it is too soon to involve SnapshotManager at this very early
stages of a node boot sequence. The stuff it needs to scan directories for
snapshots (iterating over all Keyspaces and so on) is just not there yet.
In this new implementation, we indeed get rid of ephemerals too, but it is done
a little bit later (still part of the booting sequence), just after all is
initialised properly for SnapshotManager. Removal of ephemerals is done as
one-time job and SnapshotManager is started in such a way that periodic
checking for expired snapshots is not in effect yet. We are resuming this later
in the booting sequence which will be obvious by a respective reader of the
patch.
Also, this patch also introduces an interesting problem on upgrade path;
imagine this scenario.
# I am running 4.0 where an ephemeral snapshot is taken and it is stored on
disk with ephemeral marker file
# this node on 4.0 is turned off, so ephemeral snapshot still lives on disk
because it is meant to be removed on next startup
# I upgrade this node to 4.1 (or whatever version this change will be in), but
now we are expecting to have SnapshotManifest
# Since there is a manifest file from times when ephemeral flag was not
introduced yet and marker file is not there anymore, by creating
SnapshotManifest, it will be evaluated as that snapshot is _not_ ephemeral
# Since it is not ephemeral anymore, we just "promoted" a snapshot from
ephemeral to normal one which will be never removed
I think that this problem has to be addressed by writing ephemeral marker
files, as it is done now, in paralel with introducing this flag into
SnapshotManifest so once a user is on 4.0 and he wants to upgrade to 4.1,
ephemeral marker files will be still taken into account and removed on the
startup, it is just that any new ephemeral snapshot will not have this marker
file and flag in the manifest will b persisted instead of that.
In the next version, like 4.2, we will get rid of marker file logic completely
because 4.1 will create all ephemeral snapshots with a flag in the manifest
only.
> Remove ephemeral snapshot marker file and introduce a flag to SnapshotManifest
> ------------------------------------------------------------------------------
>
> Key: CASSANDRA-16911
> URL: https://issues.apache.org/jira/browse/CASSANDRA-16911
> Project: Cassandra
> Issue Type: Improvement
> Components: Local/Other
> Reporter: Stefan Miklosovic
> Assignee: Stefan Miklosovic
> Priority: Normal
>
> While creating an ephemeral snapshot, a marker file is created at disk in a
> respective snapshot directory. This is not necessary anymore as we have
> introduced SnapshotManifest in CASSANDRA-16789 so we can move this flag
> there. By putting the information if a snapshot is ephemeral or not into
> SnapshotManifest, we simplify and "clean up" the snapshotting process and
> related codebase.
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