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From Ali Akhtar <ali.rac...@gmail.com>
Subject Re: Are Cassandra writes are faster than reads?
Date Mon, 07 Nov 2016 05:09:05 GMT
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
>

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