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From "Aaron Morton (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-2156) Compaction Throttling
Date Fri, 25 Mar 2011 01:47:06 GMT


Aaron Morton commented on CASSANDRA-2156:

Bunch of questions again as I'm trying to understand some more of whats going on. 

# if compaction_throughput_kb_per_sec is always going be megabytes should it change to MB
# Not in your changes but CompactionIterator.close() will stop closing files after the first
one fails. 
# I'm guessing most of the time the actual and target throughput will not match. How about
moving the INFO message in throttle() to the DEBUG level? Or only logging at INFO is the thread
will sleep?     
# Should there be a config setting to turn throttling on and off? Could setting compaction_throughput_kb_per_sec
to 0 disable it ?  
# For my understanding: Is there a case for making the sampling interval in CompactionIterator.getReduce()
configurable? Would we want different settings for fewer big rows vs many small rows. e.g.
two CFs where one is a secondary index for rows in the other, could be millions of cols in
one an a few in another.

I dont understand the approach to deciding what value compaction_throughput_kb_per_sec should
have. Can you add some more info and clarify if you are talking about the per CF buckets creating
during Compaction?

Final question. Would it be better to have fewer parallel compactions where each compaction
completes quickly, than more parallel compactions that take longer to complete. Assuming that
once compaction has finished read performance and disk usage may improve. If so would limiting
compaction by sizing the compaction thread pool be effective? (I guess the down side may be
starvation for some CF's) 

> Compaction Throttling
> ---------------------
>                 Key: CASSANDRA-2156
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Stu Hood
>             Fix For: 0.8
>         Attachments: 0001-Throttle-total-compaction-to-a-configurable-throughput.txt,
for-0.6-0001-Throttle-compaction-to-a-fixed-throughput.txt, for-0.6-0002-Make-compaction-throttling-configurable.txt
> Compaction is currently relatively bursty: we compact as fast as we can, and then we
wait for the next compaction to be possible ("hurry up and wait").
> Instead, to properly amortize compaction, you'd like to compact exactly as fast as you
need to to keep the sstable count under control.
> For every new level of compaction, you need to increase the rate that you compact at:
a rule of thumb that we're testing on our clusters is to determine the maximum number of buckets
a node can support (aka, if the 15th bucket holds 750 GB, we're not going to have more than
15 buckets), and then multiply the flush throughput by the number of buckets to get a minimum
compaction throughput to maintain your sstable count.
> Full explanation: for a min compaction threshold of {{T}}, the bucket at level {{N}}
can contain {{SsubN = T^N}} 'units' (unit == memtable's worth of data on disk). Every time
a new unit is added, it has a {{1/SsubN}} chance of causing the bucket at level N to fill.
If the bucket at level N fills, it causes {{SsubN}} units to be compacted. So, for each active
level in your system you have {{SubN * 1 / SsubN}}, or {{1}} amortized unit to compact any
time a new unit is added.

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