Sunday, January 26, 2014

Overwrite tmnet DNS in router DIR-615

In the past, I have read into how the site was "blocked" in Malaysia where web surfers unable to load some websites. I don't know what is the fuss about it as it certainly create a lot of talks in the social websites. The term blocked would probably be at IP level where if the access by a machine to a address of IP a.b.c.d is unreachable, then that is a really blocked. But the funny things is, some ISPs in malaysia do not actually blocked the IPs but merely make some hostname unable to resolve into IP.

So in this article, I'm gonna show you how you can get back your freedom. There are many public DNS out there. Just google public DNS would give you a lot of list. In fact, google has public DNS too. When you choose the DNS, be sure it is public trust able and speed of your machine to the DNS server is matter when you want a quick web browser able to determine the IP address quickly.

With that said, I'm gonna show you how you can overwrite the DNS entries in tmnet router DIR-615. You need to ssh to your router in order to accomplish this. I have tried many times in the web interface, though it has text box for you to specify, apparently you cannot specify to the list that you want, it always default to tmnet DNS. This is annoying!

What you need is a terminal, ssh program installed, username = operator , password = h566UniFi

After you have ssh into the router, these are all the steps you need to do.
# cat /var/etc/resolv.conf
# This file is generated by gen_resolv.php

# sed 's/' -i /var/etc/resolv.conf
# sed 's/' -i /var/etc/resolv.conf
# cat resolv.conf
# This file is generated by gen_resolv.php

So I have an internal IP because there is a internal DNS service running. would be the google public DNS.

Well, that's all you need, the site where it is unable to resolve to an IP, you can check by ping to the hostname, it should return a valid IP. That's all folks, I hope you learn something and gain back your right.

Tuesday, January 21, 2014

cassandra component on heap or off heap.

A summary of the components in cassandra that store offheap.

cassandra 1.2 and later use offheap for
bloom filter
compression offset map
row cache

cassandra 2.0 and later use offheap for
bloom filter
compression offset map
partition summary
index samples
row cache

Remember , off heap is not equal as non heap. Off-heap means the object is stored non in java heap allocated to the jvm, that is the native memory.

For example in the Bloom filter.
public static IAllocator newOffHeapAllocator(String offheap_allocator) throws ConfigurationException
if (!offheap_allocator.contains("."))
offheap_allocator = "" + offheap_allocator;
return FBUtilities.construct(offheap_allocator, "off-heap allocator");

As you can see, if you want to determine if the object is stored on heap or offheap, you can to read into the code. I tried to inspect cassandra 2.0.2 using jconsole, it only show total usage of onheap or offheap. The cassandra community suggest it is better to read NEWS.txt or their blog to read the latest change if the component move on or off heap.!topic/nosql-databases/kFHS6QfZDuE

Thursday, January 16, 2014

force sstable compaction through jconsole

Happy new year everyone, this is my first article for 2014 and as a start, it is going to be short and sweet one. :-)

I was working on this project where I have deleted the column and I remember the definition of tombstone is that, you need to run compact so that the tombstone is removed. When I run nodetool compact, this message appear below.
INFO 19:15:09,170 Nothing to compact in index1.  Use forceUserDefinedCompaction if you wish to force compaction of single sstables (e.g. for tombstone collection)

So what is this means is that, you need to have jconsole running because forceUserDefinedCompaction can only invoke through jconsole. When jconsole is connected to your cassandra daemon process, you need to navigate to the compactionmanager mbean.

Then you need to provide two value to this method as this method expected two parameters. That is the keyspace and the sstables where you wish to compact. You can see in this attachment on how I did it.

So if you ls the data directory where the sstable are store, a new sstable should be generated. Once the operation is complete, you can navigate to the data directory where the sstable is stored.

$ ls data/lucene1/
index1-hc-3-Data.db index1-hc-3-Digest.sha1 index1-hc-3-Filter.db index1-hc-3-Index.db index1-hc-3-Statistics.db snapshots

We have production machine which a single sstable size is more than 25GB. When the soft limit compaction is met (default min 4 and max 32) , in this situation, I think it will load the server if it takes place, probably best is, we forceUserDefinedCompaction on this single large sstable.