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From Karl Hiramoto <k...@hiramoto.org>
Subject Re: reducing disk usage advice
Date Mon, 14 Mar 2011 19:17:25 GMT
On 03/14/11 15:33, Sylvain Lebresne wrote:
>
> CASSANDRA-1537 is probably also a partial but possibly sufficient
> solution. That's also probably easier than CASSANDRA-1610 and I'll try
> to give it a shot asap, that had been on my todo list way too long.
>
Thanks, eager to see CASSANDRA-1610 someday.   What  I've been doing the
last day has been multiple restarts across the cluster when one node's
data/ dir gets to 150GB.  restarting cassandra brings the nodes data/
directory down to around 60GB, I see cassandra deleteing a lot of
SSTables on startup.


One question,  since I use a TTL is it safe to set GCGraceSeconds  to 0?   I don't manually
delete ever, I just rely on the TTL for deletion, so are forgotten deletes an issue?


> The rule is this. Say you think that m is a reasonable value for
> GCGraceSeconds. That is, you make sure that you'll always put back up
> failing nodes and run repair within m seconds. Then, if you always use
> a TTL of n (in your case 24 hours), the actual GCGraceSeconds that you
> should set is m - n.
>
> So putting a GCGrace of 0 in you would would be roughly equivalent to
> set a GCGrace of 24h on a "normal" CF. That's probably a bit low.
>
What do you mean by "normal"?  If I were to set GCGrace to 0 would  risk
data corruption?  Wouldn't setting GCGrace to 0 help reduce disk space
pressure?


Thanks,
Karl

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