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From Bhuvan Rawal <bhu1ra...@gmail.com>
Subject Re: High disk io read load
Date Mon, 20 Feb 2017 09:48:08 GMT
Hi Benjamin,

Yes, Read ahead of 8 would imply more IO count from disk but it should not
cause more data read off the disk as is happening in your case.

One probable reason for high disk io would be because the 512 vnode has
less page to RAM ratio of 22% (100G buff /437G data) as compared to 46%
(100G/237G). And as your avg record size is in bytes for every disk io you
are fetching complete 64K block to get a row.

Perhaps you can balance the node by adding equivalent RAM ?

Regards,
Bhuvan

On Mon, Feb 20, 2017 at 12:11 AM, Benjamin Roth <benjamin.roth@jaumo.com>
wrote:

> This is the output of sar: https://gist.github.com/anonymous/
> 9545fb69fbb28a20dc99b2ea5e14f4cd
> <https://www.google.com/url?q=https%3A%2F%2Fgist.github.com%2Fanonymous%2F9545fb69fbb28a20dc99b2ea5e14f4cd&sa=D&sntz=1&usg=AFQjCNH6r_GCSN0ZxmDx1f8xGRJPweV-EQ>
>
> It seems to me that there es not enough page cache to handle all data in a
> reasonable way.
> As pointed out yesterday, the read rate with empty page cache is ~800MB/s.
> Thats really (!!!) much for 4-5MB/s network output.
>
> I stumbled across the compression chunk size, which I always left
> untouched from the default of 64kb (https://cl.ly/2w0V3U1q1I1Y). I guess
> setting a read ahead of 8kb is totally pointless if CS reads 64kb if it
> only has to fetch a single row, right? Are there recommendations for that
> setting?
>
> 2017-02-19 19:15 GMT+01:00 Bhuvan Rawal <bhu1rawal@gmail.com>:
>
>> Hi Edward,
>>
>> This could have been a valid case here but if hotspots indeed existed
>> then along with really high disk io , the node should have been doing
>> proportionate high network io as well. -  higher queries per second as well.
>>
>> But from the output shared by Benjamin that doesnt appear to be the case
>> and things look balanced.
>>
>> Regards,
>>
>> On Sun, Feb 19, 2017 at 7:47 PM, Edward Capriolo <edlinuxguru@gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Sat, Feb 18, 2017 at 3:35 PM, Benjamin Roth <benjamin.roth@jaumo.com>
>>> wrote:
>>>
>>>> We are talking about a read IO increase of over 2000% with 512 tokens
>>>> compared to 256 tokens. 100% increase would be linear which would be
>>>> perfect. 200% would even okay, taking the RAM/Load ratio for caching into
>>>> account. But > 20x the read IO is really incredible.
>>>> The nodes are configured with puppet, they share the same roles and no
>>>> manual "optimizations" are applied. So I can't imagine, a different
>>>> configuration is responsible for it.
>>>>
>>>> 2017-02-18 21:28 GMT+01:00 Benjamin Roth <benjamin.roth@jaumo.com>:
>>>>
>>>>> This is status of the largest KS of these both nodes:
>>>>> UN  10.23.71.10  437.91 GiB  512          49.1%
>>>>> 2679c3fa-347e-4845-bfc1-c4d0bc906576  RAC1
>>>>> UN  10.23.71.9   246.99 GiB  256          28.3%
>>>>> 2804ef8a-26c8-4d21-9e12-01e8b6644c2f  RAC1
>>>>>
>>>>> So roughly as expected.
>>>>>
>>>>> 2017-02-17 23:07 GMT+01:00 kurt greaves <kurt@instaclustr.com>:
>>>>>
>>>>>> what's the Owns % for the relevant keyspace from nodetool status?
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Benjamin Roth
>>>>> Prokurist
>>>>>
>>>>> Jaumo GmbH · www.jaumo.com
>>>>> Wehrstraße 46 · 73035 Göppingen · Germany
>>>>> Phone +49 7161 304880-6 <07161%203048806> · Fax +49 7161 304880-1
>>>>> <07161%203048801>
>>>>> AG Ulm · HRB 731058 · Managing Director: Jens Kammerer
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Benjamin Roth
>>>> Prokurist
>>>>
>>>> Jaumo GmbH · www.jaumo.com
>>>> Wehrstraße 46 · 73035 Göppingen · Germany
>>>> Phone +49 7161 304880-6 <+49%207161%203048806> · Fax +49 7161 304880-1
>>>> <+49%207161%203048801>
>>>> AG Ulm · HRB 731058 · Managing Director: Jens Kammerer
>>>>
>>>
>>> When I read articles like this:
>>>
>>> http://www.doanduyhai.com/blog/?p=1930
>>>
>>> And see the word hot-spot.
>>>
>>> "Another performance consideration worth mentioning is hot-spot.
>>> Similar to manual denormalization, if your view partition key is chosen
>>> poorly, you’ll end up with hot spots in your cluster. A simple example with
>>> our *user* table is to create a materialized
>>>
>>> *view user_by_gender"It leads me to ask a question back: What can you
>>> say about hotspots in your data? Even if your nodes had the identical
>>> number of tokens this autho seems to suggesting that you still could have
>>> hotspots. Maybe the issue is you have a hotspot 2x hotspots, or your
>>> application has a hotspot that would be present even with perfect token
>>> balancing.*
>>>
>>>
>>
>
>
> --
> Benjamin Roth
> Prokurist
>
> Jaumo GmbH · www.jaumo.com
> Wehrstraße 46 · 73035 Göppingen · Germany
> Phone +49 7161 304880-6 · Fax +49 7161 304880-1
> AG Ulm · HRB 731058 · Managing Director: Jens Kammerer
>

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