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From "Slater, David M." <David.Sla...@jhuapl.edu>
Subject RE: 0% Data Cache Hit Rate
Date Tue, 12 Mar 2013 15:46:44 GMT
Thanks Keith,

I checked, and the values were all default. (cache block disabled)

Turning them on, however, turned the data cache hit rate down to single digits for all of
the data nodes. I'm guessing that the queries I am running, since they need to go through
so much data, cannot be cached well, and that the high percentages I was getting before were
due to the use metatable data cache (since that is enabled by default). 

Since data caching is disabled by default, I assume that there are downsides to using it.
Is this primarily memory footprint?

Regards,
David

-----Original Message-----
From: Keith Turner [mailto:keith@deenlo.com] 
Sent: Monday, March 11, 2013 3:02 PM
To: user@accumulo.apache.org
Subject: Re: 0% Data Cache Hit Rate

You may need to set the following property to true for the table.
This enables caching data for a table.  It defaults to false.

table.cache.block.enable

Also take a look at the following props.  These determine how much memory a tserver uses for
caching.

tserver.cache.data.size
tserver.cache.index.size

The following props enables caching rfile indexes for a table, it defaults to true.

table.cache.index.enable

Keith


On Mon, Mar 11, 2013 at 2:48 PM, Slater, David M.
<David.Slater@jhuapl.edu> wrote:
> Hi,
>
>
>
> I have a four-node setup, and I'm running some intensive query 
> operations that need to go through all of the rows (though only one or 
> two column families). While I don't expect this to be fast by any 
> means, I wanted to make sure that I had a decent baseline before 
> comparing this to more indexed versions of querying. Here is the 
> problem: Two of my nodes have very low data cache hit rates, wand I 
> assume that this would greatly impact the query efficiency. Is this correct?
>
>
>
> All four of my nodes have a 99% index cache hit rate, but the data 
> cache hit rates are:
>
> Node 1: 96%
>
> Node 2: 95%
>
> Node 3: 67%
>
> Node 4: 0%
>
> (All four are data nodes; the name node is #1)
>
>
>
> I'm not seeing any warnings or errors in the logs, and I couldn't find 
> much online about it, so I thought I would check here. Does anyone 
> have a suggestion as for how to fix it? Could this be related to the 
> system swappiness at all? (I currently have swappiness set to 0.)
>
>
>
> Thanks for the help,
> David Slater

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