On Dec 4, 2007, at 9:37 AM, Dennis Kubes wrote:
Trey Spiva wrote:
According to a hadoop tutorial (http://wiki.apache.org/nutch/
NutchHadoopTutorial) on wiki,
"you don't want to search using DFS, you want to search using
local filesystems. Once the index has been created on the DFS
you can use the hadoop copyToLocal command to move it to the local
file system as such" ... "Understand that at this point we are not
using the DFS or MapReduce to do the searching, all of it is on a
local machine".
So my understanding is that hadoop is only good for batch index
building, and is not proper for incremental index building and
search. Is this true?
That is correct. DFS for batch processing and MapReduce jobs.
Local servers (disks) for serving indexes. Even better put local
indexes (not segments, just indexes) in RAM.
In the NutchHadoopTutorial it says "the directory which it points to
should contain not just the index directory but also the linkdb,
segments, etc. All of these different databases are used by the
search. This is why we copied over the crawled directory and not just
the index directory."
If I understand your comment correctly, you are saying to not copy
the linkdb, and segments data just the index directory. Is that
correct? I think this is the source of my confusion, because it
sounds like the entire crawl data needs to be copied to each search
machine.
The reason I am asking is that when I read the article ACM article
by Mike Cafarella and Doug Cutting, to me it sounded like the
concern was to make the index structures fit in the primary
memory, not the entire crawled database. Did I miss understand
the ACM article?
No, what they are saying is the more pages per index per machine on
hard disk the slower the search. Keeping the main indexes, but not
the segments which hold raw page content, in RAM can speed up
search significantly.
One way to do this if you are running on linux is to create a
tempfs (which is ram) and then mount the filesystem in the ram.
Then your index acts normally to the application but is essentially
served from Ram. This is how we server the Nutch lucene indexes on
our web search engine (www.visvo.com) which is ~100M pages. Below
is how you can achieve this, assuming your indexes are in /path/to/
indexes:
mv /path/to/indexes /path/to/indexes.dist
mkdir /path/to/indexes
cd /path/to
mount -t tmpfs -o size=2684354560 none /path/to/indexes
rsync --progress -aptv indexes.dist/* indexes/
chown -R user:group indexes
This would of course be limited by the amount of RAM you have on
the machine. But with this approach most searches are sub-second.
Thanks for the information.
Dennis Kubes
