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From "Shawn Heisey" <>
Subject Re: Solr Replication is not Possible on RAMDirectory?
Date Tue, 06 Nov 2012 01:21:23 GMT

> Shawn Heisey-4 wrote
>> ... transparently mapping the files on disk to a virtual memory space
>> and
>> using excess RAM to cache that data and make it fast.  If you have
>> enough extra memory (disk cache) to fit the entire index, the OS will
>> never have to read any part of the index from disk more than once....
> so for disk cache, are there any disks with 1 gigs or more of caches? if
> am
> not wrong there are mostly 16 or 32mb cache disks around (or i am checking
> the wrong stuff? ) if so, that amount definitely too small...

I am not talking about the cache on the actual disk drive, or even cache
on your hard drive controller. I am talking about the operating system
using RAM, specifically RAM not being used by programs, to cache data on
your hard drive. All modern operating systems do it, even the one made in
Redmond that people love to hate.

If you have 16 GB of RAM and all your programs use up 4.5 GB, you can
count on the OS using at least another half GB, so you have about 11 GB
left. The OS is going to put data that it reads and writes to/from your
disk in this space. If you start up another program that wants 2GB, the OS
will simply throw away 2 GB of data in its cache (it's still on the disk,
after all) and give that RAM to the new program.

Solr counts on this OS capability for good performance.


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