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https://issues.apache.org/jira/browse/CASSANDRA-2156?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13013867#comment-13013867
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Stu Hood commented on CASSANDRA-2156:
-------------------------------------
1. Fixed
2. Added and used {{FileUtils.close(Collection<Closeable>)}}
3. targetBytesPerMS only changes when the number of active threads changes: it
leads to nice (imo) periodic feedback of running compactions in the log when
compactions start or finish
4. Assuming compaction multithreading makes it in, throttling should never be
disabled... for someone who really wants to disable it, setting it to a high
enough value that it never kicks in should be sufficient?
5. Maybe... but dynamically adjusting the frequency at which we throttle and
update {{bytesRead}} would probably be better to do in another thread?
----
Regarding the approach to setting compaction_throughput_mb_per_sec: each bucket
probably contains {{MIN_THRESHOLD}} times more data than the previous bucket,
and needs to be compacted {{1 / MIN_THRESHOLD}} times as often (see the math in
the description). This means that the number of buckets influences how fast you
need to compact, and that each additional bucket adds a linear amount of
necessary throughput (+ 1x your flush rate). Therefore, if you have 15 bucket
levels, and you are flushing {{1 MB/s}}, you need to compact at {{1 MB/s * 15}}.
As an example: with {{MIN_THRESHOLD=2}}, each bucket is twice is large as the
previous. Say that we have 4 levels (buckets of sizes 1, 2, 4, 8) and that we
need a compaction in the largest bucket. The amount of data that needs to be
compacted in that bucket will be equal to 1 less than the sum of the sizes of
all the other buckets (1 + 2 + 4 == 8 - 1). So, ideally we would be able to
compact those 8 units in the time it _exactly_ the time it takes for 1 more
unit to be flushed, and for the compactions of the other buckets to trickle up
and refill the largest bucket. Pheew?
CASSANDRA-2171 will allow us to calculate the flush rate, which we can then
multiply by the count of buckets (note... one tiny missing piece is determining
how many buckets are "empty": an empty bucket is not created in the current
approach).
----
> Final question. Would it be better to have fewer parallel compactions
As a base case, with no parallelism at all, you _will_ fall behind on
compaction, because every new bucket is a chance to compact. It's a fundamental
question, but I haven't thought about it... sorry.
> Compaction Throttling
> ---------------------
>
> Key: CASSANDRA-2156
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2156
> 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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