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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Memtable tuning in 1.0 and higher
Date Thu, 28 Jun 2012 16:44:33 GMT
[moving to user list]

1.0 doesn't care about throughput or op count anymore, only whether
the total memory used by the *currrent* data in the memtables has
reached the global limit.  So, it automatically doesn't count
"historical" data that's been overwritten in the current memtable.

So, you may want to increase the memory allocated to memtables... or
you may be seeing flushes forced by the commitlog size cap, which you
can also adjust.

But, the bottom line is I'd consider flushing every 5-6 minutes to be
quite healthy; since the amount of "time flushing" : "time not
flushing" ratio is quite small, reducing it further is going to give
you negligible benefit (in exchange for longer replay times.)

On Thu, Jun 28, 2012 at 5:09 AM, Joost van de Wijgerd
<jwijgerd@gmail.com> wrote:
> Hi,
>
> I work for eBuddy, We've been using Cassandra in production since 0.6
> (using 0.7 and 1.0, skipped 0.8) and use it for several Use Cases. One of
> our uses is to persist our sessions.
>
> Some background, in our case sessions are long lived, we have a mobile
> messaging platform where sessions are essentially eternal. We use cassandra
> as a system of record for our session so in case of scale out or fail over
> we can quickly load the session state again. We use protocolbuffers to
> serailize
> our data into a byte buffer and then store this as a column value in a
> (wide) row. We use a partition based approach to scale and each partition
> has it's own
> row in cassandra. Each session is mapped to a partition and stored in a
> column in this row.
>
> Every time there is a change in the session (i.e. message add, acked etc)
> we schedule the session to be flushed to cassandra. Every x seconds we flush
> the dirty sessions. So there are a serious number of (over)writes going on
> and not that many reads (unless there is a failover situation or we scale
> out). This
> is using one of the strengths of cassandra.
>
> In versions 0.6 and 0.7 it was possible to control the memtable settings on
> a CF basis. So for this particular CF we would set the throughput really
> high since there
> are a huge number of overwrites. In the same cluster we have other CFs that
> have a different load pattern.
>
> Since we moved to version 1.0 however, it has become almost impossible to
> tune our system for this (mixed) workload. Since we now have only two knobs
> to turn (the size
> of the commit log and the total memtable size) and you have introduced the
> liveRation calculation. While this works ok for most workloads, our
> persistent session store
> is really hurt by the fact that the liveRatio cannot be lower than 1.0
>
> We generally have an actual liveRatio of 0.025 on this CF due to the huge
> number of overwrites. We are now artificially tuning up the total memtable
> size but this interferes
> with our other CFs who have a different workload. Due to this, our
> performance has degraded quite a bit since on our 0.7 version we had our
> session CF tuned so that
> it would flush only once an hour, thus absorbing way more overwrites, thus
> having to do less compactions and on a failover scenario most request could
> be served straight
> from the memtable (since we are doing since column reads there). Currently
> we flush every 5 to 6 minutes under moderate load, so 10 times worse. This
> is with the s
> same heap setting etc.
>
> Would you guys consider allowing lower values than 1.0 for the liveRatio
> calculation? This would help us a lot. Perhaps make it a flag so it can be
> turned on and off? Ideally
> I would like the possibility back to tune on a CF by CF basis, this could
> be a special setting that needs to be enabled for power users. The default
> being what's there now.
>
> Also, in the current version the live ration can never adjust downwards, I
> see you guys have already made a fix for this in 1.1 but I have not seen it
> on the 1.0 branch.
>
> Let me know what you think
>
> Kind regards,
>
> Joost
>
> --
> Joost van de Wijgerd
> joost.van.de.wijgerd@Skype
> http://www.linkedin.com/in/jwijgerd



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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