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From Robert Coli <rc...@eventbrite.com>
Subject Re: Moving SSTables from one disk to another
Date Fri, 10 Apr 2015 23:13:18 GMT
On Fri, Apr 10, 2015 at 4:00 PM, Roman Tkachenko <roman@mailgunhq.com>
wrote:

> * Can I just move some SSTables data files from "sstables2" to "sstables1"
> which has much more free disk space? Will Cassandra start fine after that
> and not lose any data?
>

Cassandra generally discovers files in its data directories and treats them
as legitimate files. I do not have specific knowledge of JBOD behavior
here, but I would presume it would be the same.


> * Provided multiple data dirs, should Cassandra distribute data equally
> between them? In what I'm observing this is almost always not true. On that
> particular node I mentioned above the difference is huge: 4% occupied disk
> space for "sstables1" and 87% for "sstables2"; on other nodes the situation
> is a little better but still not 50/50.
>

No, and especially not when using Size Tiered Compaction.

I honestly wonder why people think JBOD is a useful feature for Cassandra.
You don't really want to continue to operate a node that has lost half of
its data, and managing multiple data directories seems relatively likely to
be more trouble than it's worth. You have a distributed, replicated
database... just replace nodes when they fail. Anyone care to set me
straight about the amazing benefits they see which make the costs
worthwhile?

=Rob

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