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From Ben Bromhead <...@instaclustr.com>
Subject Re: Are Cassandra writes are faster than reads?
Date Mon, 07 Nov 2016 00:31:56 GMT
To add some flavor as to how the commitlog implementation is so quick.

It only flushes to disk every 10s by default. So writes are effectively
done to memory and then to disk asynchronously later on. This is generally
accepted to be OK, as the write is also going to other nodes.

You can of course change this behavior to flush on each write or to skip
the commitlog altogether (danger!). This however will change how "safe"
things are from a durability perspective.

On Sun, Nov 6, 2016, 12:51 Jeff Jirsa <jeff.jirsa@crowdstrike.com> wrote:

> Cassandra writes are particularly fast, for a few reasons:
>
>
>
> 1)       Most writes go to a commitlog (append-only file, written
> linearly, so particularly fast in terms of disk operations) and then pushed
> to the memTable. Memtable is flushed in batches to the permanent data
> files, so it buffers many mutations and then does a sequential write to
> persist that data to disk.
>
> 2)       Reads may have to merge data from many data tables on disk.
> Because the writes (described very briefly in step 1) write to immutable
> files, updates/deletes have to be merged on read – this is extra effort for
> the read path.
>
>
>
> If you don’t do much in terms of overwrites/deletes, and your partitions
> are particularly small, and your data fits in RAM (probably mmap/page cache
> of data files, unless you’re using the row cache), reads may be very fast
> for you. Certainly individual reads on low-merge workloads can be < 0.1ms.
>
>
>
> -          Jeff
>
>
>
> *From: *Vikas Jaiman <er.vikasjaiman@gmail.com>
> *Reply-To: *"user@cassandra.apache.org" <user@cassandra.apache.org>
> *Date: *Sunday, November 6, 2016 at 12:42 PM
> *To: *"user@cassandra.apache.org" <user@cassandra.apache.org>
> *Subject: *Are Cassandra writes are faster than reads?
>
>
>
> Hi all,
>
>
>
> Are Cassandra writes are faster than reads ?? If yes, why is this so? I am
> using consistency 1 and data is in memory.
>
>
>
> Vikas
>
-- 
Ben Bromhead
CTO | Instaclustr <https://www.instaclustr.com/>
+1 650 284 9692
Managed Cassandra / Spark on AWS, Azure and Softlayer

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