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From Takenori Sato <ts...@cloudian.com>
Subject Re: Recommended amount of free disk space for compaction
Date Sat, 30 Nov 2013 02:21:45 GMT
Hi,

> If Cassandra only compacts one table at a time, then I should be safe if
I keep as much free space as there is data in the largest table. If
Cassandra can compact multiple tables simultaneously, then it seems that I
need as much free space as all the tables put together, which means no more
than 50% utilization.

Based on your configuration. 1 per CPU core by default. See
concurrent_compactors for details.

> Also, what happens if a node gets low on disk space and there isn’t
enough available for compaction?

A compaction checks if there's enough disk space based on its estimate.
Otherwise, it won't get executed.

> Is there a way to salvage a node that gets into a state where it cannot
compact its tables?

If you carefully run some cleanups, then you'll get some room based on its
new range.


On Fri, Nov 29, 2013 at 12:21 PM, Robert Wille <rwille@fold3.com> wrote:

> I’m trying to estimate our disk space requirements and I’m wondering about
> disk space required for compaction.
>
> My application mostly inserts new data and performs updates to existing
> data very infrequently, so there will be very few bytes removed by
> compaction. It seems that if a major compaction occurs, that performing the
> compaction will require as much disk space as is currently consumed by the
> table.
>
> So here’s my question. If Cassandra only compacts one table at a time,
> then I should be safe if I keep as much free space as there is data in the
> largest table. If Cassandra can compact multiple tables simultaneously,
> then it seems that I need as much free space as all the tables put
> together, which means no more than 50% utilization. So, how much free space
> do I need? Any rules of thumb anyone can offer?
>
> Also, what happens if a node gets low on disk space and there isn’t enough
> available for compaction? If I add new nodes to reduce the amount of data
> on each node, I assume the space won’t be reclaimed until a compaction
> event occurs. Is there a way to salvage a node that gets into a state where
> it cannot compact its tables?
>
> Thanks
>
> Robert
>
>

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