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https://issues.apache.org/jira/browse/CASSANDRA-3620?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13169761#comment-13169761
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Dominic Williams commented on CASSANDRA-3620:
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Ok I got it and +1 on that idea. Abandon GCSeconds and simply kill of
tombstones created before repair when it runs successfully (presumably on a
range-by-range basis)
* Improved performance through reduced tombstone load
* No risk of data corruption if repair not run
That would be a very cool first step to optimize this
I think a reaper system would still be well worthwhile though, although this
feature would take some pressure off. There is still the issue of tombstone
build up between repairs, which means performance can vary (or actually,
degrade) between invocations plus there are still the load spikes from repair
itself
I guess I'm on the sharp end of this - we have several column families where
columns represent game objects or messages owned by users where there is a high
delete and insert load. Various operations need to perform slices of user rows
and these can get much slower as tombstones build up, so GCSeconds has been
brought right down, but this leads to the constant pain of "omg how long left
before need to run repair or increase GCSeconds" etc.. improving repair would
remove the Sword of Damocles thing but we'd still need to run it regularly and
performance wouldn't be as consistent it could be with constant background
reaping
> Proposal for distributed deletes - use "Reaper Model" rather than GCSeconds
> and scheduled repairs
> -------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-3620
> URL: https://issues.apache.org/jira/browse/CASSANDRA-3620
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Dominic Williams
> Labels: GCSeconds,, deletes,, distributed_deletes,,
> merkle_trees, repair,
> Original Estimate: 504h
> Remaining Estimate: 504h
>
> Here is a proposal for an improved system for handling distributed deletes.
> h2. The Problem
> There are various issues with repair:
> * Repair is expensive anyway
> * Repair jobs are often made more expensive than they should be by other
> issues (nodes dropping requests, hinted handoff not working, downtime etc)
> * Repair processes can often fail and need restarting, for example in cloud
> environments where network issues make a node disappear
> from the ring for a brief moment
> * When you fail to run repair within GCSeconds, either by error or because of
> issues with Cassandra, data written to a node that did not see a later delete
> can reappear (and a node might miss a delete for several reasons including
> being down or simply dropping requests during load shedding)
> * If you cannot run repair and have to increase GCSeconds to prevent deleted
> data reappearing, in some cases the growing tombstone overhead can
> significantly degrade performance
> Because of the foregoing, in high throughput environments it can be very
> difficult to make repair a cron job. It can be preferable to keep a terminal
> open and run repair jobs one by one, making sure they succeed and keeping and
> eye on overall load to reduce system impact. This isn't desirable, and
> problems are exacerbated when there are lots of column families in a database
> or it is necessary to run a column family with a low GCSeconds to reduce
> tombstone load (because there are many write/deletes to that column family).
> The database owner must run repair within the GCSeconds window, or increase
> GCSeconds, to avoid potentially losing delete operations.
> It would be much better if there was no ongoing requirement to run repair to
> ensure deletes aren't lost, and no GCSeconds window. Ideally repair would be
> an optional maintenance utility used in special cases, or to ensure ONE reads
> get consistent data.
> h2. "Reaper Model" Proposal
> # Tombstones do not expire, and there is no GCSeconds
> # Tombstones have associated ACK lists, which record the replicas that have
> acknowledged them
> # Tombstones are only deleted (or marked for compaction) when they have been
> acknowledged by all replicas
> # When a tombstone is deleted, it is added to a fast "relic" index of MD5
> hashes of cf-key-name[-subName]-ackList. The relic index makes it possible
> for a reaper to acknowledge a tombstone after it is deleted
> # Background "reaper" threads constantly stream ACK requests to other nodes,
> and stream back ACK responses back to requests they have received (throttling
> their usage of CPU and bandwidth so as not to affect performance)
> # If a reaper receives a request to ACK a tombstone that does not exist, it
> creates the tombstone and adds an ACK for the requestor, and replies with an
> ACK
> NOTES
> * The existence of entries in the relic index do not affect normal query
> performance
> * If a node goes down, and comes up after a configurable relic entry timeout,
> the worst that can happen is that a tombstone that hasn't received all its
> acknowledgements is re-created across the replicas when the reaper requests
> their acknowledgements (which is no big deal since this does not corrupt data)
> * Since early removal of entries in the relic index does not cause
> corruption, it can be kept small, or even kept in memory
> * Simple to implement and predictable
> h3. Planned Benefits
> * Operations are finely grained (reaper interruption is not an issue)
> * The labour & administration overhead associated with running repair can be
> removed
> * Reapers can utilize "spare" cycles and run constantly in background to
> prevent the load spikes and performance issues associated with repair
> * There will no longer be the threat of corruption if repair can't be run for
> some reason (for example because of a new adopter's lack of Cassandra
> expertise, a cron script failing, or Cassandra bugs preventing repair being
> run etc)
> * Deleting tombstones earlier, thereby reducing the number involved in query
> processing, will often dramatically improve performance
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