[Hadoop-common-user] Split control in Lzo index

On Thu, Jun 23, 2011 at 1:59 PM, Shi Yu wrote:

Hi,

My specific question is: is it possible to control the split of Lzo files
by customize the Lzo index files?

The background of the problem is:

I have a file which has the following format

key1 value1
key1 value2
key2 value3
key2 value4
...

Its size in plain text before compression is 11 M. After Lzo compression,
the size is 681 K. I tried this on two formats: Text format and Sequence
format with block compression. They are almost the same.

However, when I join the same keys together and reformat the file as

key1 value1 value2
key2 value3 value4
...

The size before compression is of course more or less the same, 11M. But
after Lzo compression, the size is 4.8 M. My guess is: maybe the Lzo
compression algorithm could compress a lot of similar values in the first
format, whereas in the second format the concatenation of multiple values
are less likely to be identical, therefore the compression rate decreases.

So, again my question is, if I would like to keep the file in the first
format, I would prohibit mapper to split the file within the same key. For
example, all "key1" should go to the same mapper. Is it doable on a Lzo
file? Because the split behavior of Lzo files relies on the index files, is
there anyway to control the split by customizing the Lzo index files?

BTW, when using the second format, I found that bzip2 has better
compression rate than Lzo (2.1 M). Did I made any mistake when using Lzo
compression?

Thanks!

Best Regards,

Shi

Shi,
bzip compresses much better than lzo. It is also significantly more
expensive (we are talking orders of magnitude) than LZO, both on compression
and decompression.

As for your question regarding custom splits -- LzoIndex does not support
this kind of logic, as it's written to be generic and doesn't know how to
read individual records, but you can certainly customize it to fit your use
case.

D



On Thu, Jun 23, 2011 at 1:59 PM, Shi Yuwrote: