If you need to compress the db2diag and create a new one follow the steps below:

1-  Finding the db2diag.log path

at the prompting command type:

db2 get dbm cfg | grep -i diag

You will see something like this:

server :# db2 get dbm cfg | grep -i diag
 Diagnostic error capture level              (DIAGLEVEL) = 3
 Diagnostic data directory path             (DIAGPATH) = /db2/instance1/sqllib/db2dump

2 -  Going to db2diag.log path

at the promptig command type:

cd /db2/instance1/sqllib/db2dump

ls - ltr | grep -i diag

You will see something like this:

-rw-r-----   1 sample  db2iadm1    6291344 Mar 11 16:30 db2eventlog.000.crash
-rw-rw-rw-   1 sample  db2iadm1   19745333 Apr 07 15:53 db2diag.log.old.gz
-rw-rw-rw-   1 sample  db2iadm1      19580 Apr 23 06:50 sample.nfy
-rw-r-----   1 sample  db2iadm1    6291344 Jun 23 23:41 db2eventlog.000
-rw-r--r--   1 sample  db2iadm1  688961095 Jun 23 23:53 db2diag.log

3 - Creating a new one db2diag.log

at the prompting command type:

db2diag -A

You will see something like this:

/usr/local/db2/sample/sqllib/db2dump>db2diag -A
db2diag: Moving "/usr/local/db2/sample/sqllib/db2dump/db2diag.log"                     to     "/usr/local/db2/sample/sqllib/db2dump/db2diag.log_2008-06-24-21.13.31"

4-  Compressing the old db2diag.log

at the prompting command type:

compress db2diag.log