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From Ben Bromhead <...@instaclustr.com>
Subject Re: Are Cassandra writes are faster than reads?
Date Mon, 07 Nov 2016 05:44:30 GMT
They can be and it depends on your compaction strategy :)

On Sun, 6 Nov 2016 at 21:24 Ali Akhtar <ali.rac200@gmail.com> wrote:

> tl;dr? I just want to know if updates are bad for performance, and if so,
> for how long.
>
> On Mon, Nov 7, 2016 at 10:23 AM, Ben Bromhead <ben@instaclustr.com> wrote:
>
> Check out https://wiki.apache.org/cassandra/WritePathForUsers for the
> full gory details.
>
> On Sun, 6 Nov 2016 at 21:09 Ali Akhtar <ali.rac200@gmail.com> wrote:
>
> How long does it take for updates to get merged / compacted into the main
> data file?
>
> On Mon, Nov 7, 2016 at 5:31 AM, Ben Bromhead <ben@instaclustr.com> wrote:
>
> 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
>
>
> --
> Ben Bromhead
> CTO | Instaclustr <https://www.instaclustr.com/>
> +1 650 284 9692
> Managed Cassandra / Spark on AWS, Azure and Softlayer
>
>
> --
Ben Bromhead
CTO | Instaclustr <https://www.instaclustr.com/>
+1 650 284 9692
Managed Cassandra / Spark on AWS, Azure and Softlayer

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