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https://issues.apache.org/jira/browse/CASSANDRA-7872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14159474#comment-14159474
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Pavel Yaskevich commented on CASSANDRA-7872:
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That's exactly my point, "certain point in time" could be only when Full GC is
performed as an fallback for CMS (exactly what we have observed previously) and
that point could take weeks to happen which means that system will, most
luckily, run out of disk space even before phantom references are determined to
be cleaned, which brings us back to the point on relying on implementation
detail (in our cases that it's going to happen on minor collection or CMS) with
no execution guarantees which gives everybody a false impression of safely
where there is none.
> ensure compacted obsolete sstables are not open on node restart and nodetool
> refresh, even on sstable reference miscounting or deletion tasks are failed.
> ---------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-7872
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7872
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Oleg Anastasyev
> Assignee: Oleg Anastasyev
> Fix For: 2.0.11
>
> Attachments: EnsureNoObsoleteSSTables-7872-v2.0.txt
>
>
> Since CASSANDRA-4436 compacted sstables are no more marked with
> COMPACTED_MARKER file. Instead after they are compacted, DataTracker calls
> SSTableReader.markObsolete(), but the actual deletion is happening later on
> SSTableReader.releaseReference().
> This reference counting is very fragile, it is very easy to introduce a
> hard-to-catch and rare bug, so this reference count never reaches 0 ( like
> CASSANDRA-6503 for example )
> This means, that very rarely obsolete sstable files are not removed from disk
> (but are not used anymore by cassandra to read data).
> If more than gc grace time has passed since sstable file was not removed from
> disk and operator issues either nodetool refresh or just reboots a node,
> these obsolete files are being discovered and open for read by a node. So
> deleted data is resurrected, being quickly spread by RR to whole cluster.
> Because consequences are very serious (even a single not removed obsolete
> sstable file could render your data useless) this patch makes sure no
> obsolete sstable file can be open for read by:
> 1. Removing sstables on CFS init analyzing sstable generations (sstable is
> removed, if there are another sstable, listing this as ancestor)
> 2. Reimplementing COMPACTED_MARKER file for sstable. This marker is created
> as soon as markObsolete is called. This is neccessary b/c generation info can
> be lost (when sstables compact to none)
> 3. To remove sstables sooner then restart - reimplemented the good old GC
> phantom reference queue as well.
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