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From kurt greaves <k...@instaclustr.com>
Subject RE: Massive deletes -> major compaction?
Date Fri, 22 Sep 2017 11:44:09 GMT
yes, yes, yes.
A compaction on a single sstable will only get rid of tombstones if there
is no live data that tombstone shadows in any other sstable.

to actually remove data with a tombstone the compaction needs to include
other sstables that contain data the tombstone covers, and the tombstone
will only finally be removed if all that data is removed AND GCGS has
passed.

Note that thugs could happen gradually over a long period of time for a
specific tombstone, it doesn't need to be in a single compaction with all
the other data at once, it will remove the data incrementally through
multiple compactions.

On 22 Sep. 2017 06:45, "Durity, Sean R" <SEAN_R_DURITY@homedepot.com> wrote:

So, let me make sure my assumptions are correct (and let others learn as
well):



-          A major compaction would read all sstables at once (ignoring the
max_threshold), thus the potential for needing double the disk space (of
course if it wrote 30% less, it wouldn’t be double…)

-          Major compaction would leave one massive sstable, that wouldn’t
get automatically compacted for a long time

-          A user-defined compaction on 1 sstable would not evict any
tombstoned data that is in any other sstable (like a newer one with the
deletes…). It would only remove data if the tombstone is already in the
same sstable.





Sean Durity



*From:* Jeff Jirsa [mailto:jjirsa@gmail.com]
*Sent:* Thursday, September 21, 2017 2:51 PM
*To:* user@cassandra.apache.org
*Subject:* Re: Massive deletes -> major compaction?



The major compaction is most efficient but can temporarily double (nearly)
disk usage - if you can afford that, go for it.



Alternatively you can do a user-defined compaction on each sstable in
reverse generational order (oldest first) and as long as the data is
minimally overlapping it’ll purge tombstones that way as well - takes
longer but much less disk involved.





-- 

Jeff Jirsa




On Sep 21, 2017, at 11:27 AM, Durity, Sean R <SEAN_R_DURITY@homedepot.com>
wrote:

Cassandra version 2.0.17 (yes, it’s old – waiting for new hardware/new OS
to upgrade)



In a long-running system with billions of rows, TTL was not set. So a
one-time purge is being planned to reduce disk usage. Records older than a
certain date will be deleted. The table uses size-tiered compaction.
Deletes are probably 25-40% of the complete data set. To actually recover
the disk space, would you recommend a major compaction after the
gc_grace_seconds time? I expect compaction would then need to be scheduled
regularly (ick)…



We also plan to re-insert the remaining data with a calculated TTL, which
could also benefit from compaction.





Sean Durity


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