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strange grep or tail behaviour

Why does
 tail -f logfile | grep keyword | grep -v exclude-keyword
react slow?

In other words, if I do a "tail -f logfile | grep keyword" I immediately
see results.

But if I pipe the output into another grep, then the results are delayed
until some buffer is filled up or a certain amount of lines is sent.

For Example:

$ tail -f ~/logs/access.log | grep postfix               
10.1.175.3 - - [01/May/2001:12:04:40 -0700] "GET
/software/virtualmail-pop3d/postfix.html HTTP/1.0" 304 - "-" "Mozilla/4.0
(compatible; MSIE 5.5; Windows 98; Win 9x 4.90)"
10.160.213.200 - - [01/May/2001:12:05:02 -0700] "GET
/software/virtualmail-pop3d/postfix.html HTTP/1.0" 200 7146
"-" "Lynx/2.8.2rel.1 libwww-FM/2.14"
10.160.213.200 - - [01/May/2001:12:07:53 -0700] "GET
/software/virtualmail-pop3d/postfix.html HTTP/1.0" 200 7146
"-" "Lynx/2.8.2rel.1 libwww-FM/2.14"
10.160.213.200 - - [01/May/2001:12:07:55 -0700] "GET
/software/virtualmail-pop3d/postfix.html HTTP/1.0" 200 7146
"-" "Lynx/2.8.2rel.1 libwww-FM/2.14"

But when I do the following test:

$ tail -f ~/logs/access.log | grep postfix | grep postfix

Or:
$ tail -f ~/logs/access.log | grep GET | grep -v BOGUS

$ tail -f ~/logs/access.log | grep GET | grep G

I get no results. I am watching the logs on another console and I know
they are coming in.

It does work if I use "cat" instead of "tail -f"

So then I wait a while (maybe ten matches sent through) and it finally
works.

What causes this behaviour?

What do I need to do to see the matches at once?

For example,
 tail -f access.log | grep GET
instantly shows many matches
But
 tail -f access.log | grep GET
doesn't display anything (unless I wait awhile for several future entries).

By the way, I am using:
 grep (GNU grep) 2.4.2
 tail (GNU textutils) 2.
 GNU bash, version 2.03.0(1)-release (i386-pc-linux-gnu)

But I have now also seen this issue with different OSs and greps and
tails.

  Jeremy C. Reed
  http://www.reedmedia.net/
  http://bsd.reedmedia.net/  -- BSD news and resources
  http://www.isp-faq.com/    -- find answers to your questions
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