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From David Alves <davidral...@gmail.com>
Subject Re: Question about memory_limit_hard_bytes and
Date Mon, 24 Apr 2017 17:45:53 GMT
Hi Jason

  memory_limit_hard_bytes refers to all the memory consumed by the tablet
server. It has an effect in many things but arguably the biggest effect is
that the tablet server will reject all writes when memory consumption
reaches this limit.
  block_cache_capacity_mb dictates how much memory should be saved for the
block cache. The block cache caches blocks that are read from disk, thus
speeding up scans that hit cached data. The block cache also has some
effect on writes, by allowing to cache bloomfilter blocks, which we lookup
to make sure that rows are unique. The page cache caches previously fetched
disk pages, but is managed by the OS and thus out of Kudu's control.


On Mon, Apr 24, 2017 at 8:24 AM, Jason Heo <jason.heo.sde@gmail.com> wrote:

> Hello.
> I'm using Apache Kudu 1.3.0.
> I found some explanation in manual and cdh about memory_limit_hard and
> block_cache_capacity But I couldn't understand what are they exactly used
> for, what is stored at block_cache, and what's the meaning of hard limit of
> memory?
> In Kudu Configuration Reference:
>    - *memory_limit_hard_bytes
>    <https://kudu.apache.org/docs/configuration_reference.html#kudu-tserver_memory_limit_hard_bytes>*:
>    Maximum amount of memory this daemon should use, in bytes. A value of 0
>    autosizes based on the total system memory. A value of -1 disables all
>    memory limiting.
>    - *block_cache_capacity_mb
>    <https://kudu.apache.org/docs/configuration_reference.html#kudu-tserver_block_cache_capacity_mb>*:
>    block cache capacity in MB
> In Cloudera Manager's help:
>    - *memory_limit_hard_bytes*: Maximum amount of memory that the Kudu
>    Tablet Server will use before it starts rejecting all incoming writes.
>    - *block_cache_capacity_mb*: Maximum amount of memory allocated to the
>    Kudu Tablet Server's block cache.
> Is hard_limit related with insert/update/delete and is not related to
> performance of read? Does block_cache store tablet data? Then what's the
> relationship with page cache.
> After watching Comcast presentation, I was surprised because they are
> using memory_limit_hard=4G, block_cache=512MB.
> Regards,
> Jason

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