Hi Simon,

since Cassandra 2.2, anticompaction is performed in all types of repairs,
except subrange repair.
Given that you have some very big SSTables, the temporary space used by
anticompaction (which does the opposite of compaction : read one sstable,
output two sstables) will impact your disk usage while it's running. It
will reach a peak when they are close to completion.
The anticompaction that is reported by compactionstats is currently using
an extra 147GB*[compression ratio]. So with a compression ratio of 0.3 for
example, that would be 44GB that will get reclaimed shortly after the
anticompaction is over.

You can check the current overhead of compaction by listing temporary
sstables : *tmp*Data.db

It's also possible that you have some overstreaming that occurred during
your repair, which will increase the size on disk until it gets compacted
away (over time).
You should also check if you don't have snapshots sticking around by
running "nodetool listsnapshots".

Now, you're mentioning that you ran repair to evict tombstones. This is not
what repair does, and tombstones are evicted through compaction when they
meet the requirements (gc_grace_seconds and all the cells of the partition
involved in the same compaction).
If you want to optimize your tombstone eviction, especially with STCS, I
advise to turn on unchecked_tombstone_compaction, which will allow single
sstables compactions to be triggered by Cassandra when there is more than
20% of estimated droppable tombstones in an SSTable.
You can check your current droppable tombstone ratio by running
sstablemetadata on all your sstables.
A command like the following should do the trick (it will print out min/max
timestamps too) :

for f in *Data.db; do meta=$(sudo sstablemetadata $f); echo -e "Max:"
$(date --date=@$(echo "$meta" | grep Maximum\ time | cut -d" "  -f3| cut -c
1-10) '+%m/%d/%Y') "Min:" $(date --date=@$(echo "$meta" | grep Minimum\
time | cut -d" "  -f3| cut -c 1-10) '+%m/%d/%Y') $(echo "$meta" | grep
droppable) ' \t ' $(ls -lh $f | awk '{print $5" "$6" "$7" "$8" "$9}'); done
| sort

Check if the 20% threshold is high enough by verifying that newly created
SSTables don't already reach that level, and adjust accordingly if it's the
case (for example raise the threshold to 50%).

To activate the tombstone compactions, with a 50% droppable tombstone
threshold, perform the following statement on your table :

ALTER TABLE cargts.eventdata WITH compaction =
'unchecked_tombstone_compaction':'true', 'tombstone_threshold':'0.5'}

Picking the right threshold is up to you.
Note that tombstone compactions running more often will use temporary space
as well, but they should help evicting tombstones faster if the partitions
are contained within a single SSTable.

If you are dealing with TTLed data and your partitions spread over time,
I'd strongly suggest considering TWCS instead of STCS which can remove
fully expired SSTables much more efficiently.


On Fri, Jan 5, 2018 at 7:43 AM wxn...@zjqunshuo.com <wxn...@zjqunshuo.com>

> Hi All,
> In order to evict tombstones, I issued full repair with the command
> "nodetool -pr -full". Then the data load size was indeed decreased by 100G
> for each node by using "nodetool status" to check. But the actual disk
> usage increased by 500G for each node. The repair is still ongoing and
> leaving less and less disk space for me.
> From compactionstats, I see "Anticompaction after repair". Based on my
> understanding, it is for incremental repair by changing sstable metadata to
> indicate which file is repaired, so in next repair it is not going to be
> repaired. But I'm doing full repair, Why Anticompaction?
> 9e09c490-f1be-11e7-b2ea-b3085f85ccae   Anticompaction after repair     cargts 
>   eventdata    147.3 GB   158.54 GB   bytes     92.91%
> There are pare sstable files. I mean they have the same timestamp as
> below. I guess one of them or both of them should be deleted after during
> repair, but for some unknown reason, the repair process failed to delete
> them.
> -rw-r--r-- 1 root root 237G Dec 31 12:48 lb-123800-big-Data.db
> -rw-r--r-- 1 root root 243G Dec 31 12:48 lb-123801-big-Data.db
> C* version is 2.2.8 with STCS. Any ideas?
> Cheers,
> -Simon

Alexander Dejanovski

Apache Cassandra Consulting

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