Чтобы применялся профайл пользователя, в которого вы делает SU, надо писать su - db2inst1 !!!
А так вы на переменные среды никак не повлияли...

strace -fx -o /tmp/db2s.out db2set

А это так и не поставили?
db2set DB2_DIRECT_IO=OFF

Какая у вас версия debian?

root@lodge:/tmp/expc# ./db2_install
DBI1324W Support of the db2_install command is deprecated. For
more information, see the DB2 Information Center.



Default directory for installation of products - /opt/ibm/db2/V10.1

***********************************************************
Install into default directory (/opt/ibm/db2/V10.1) ? [yes/no]
yes
DB2 installation is being initialized.

Total number of tasks to be performed: 34
Total estimated time for all tasks to be performed: 986 second(s)

Task #1 start
Description: Checking license agreement acceptance
Estimated time 1 second(s)
Task #1 end

Task #2 start
Description: Base Client Support for installation with root privileges
Estimated time 3 second(s)
Task #2 end

Task #3 start
Description: Product Messages - English
Estimated time 13 second(s)
Task #3 end

Task #4 start
Description: Base client support
Estimated time 125 second(s)
Task #4 end

Task #5 start
Description: Java Runtime Support
Estimated time 127 second(s)
Task #5 end

Task #6 start
Description: Java Help (HTML) - English
Estimated time 7 second(s)
Task #6 end

Task #7 start
Description: Base server support for installation with root privileges
Estimated time 8 second(s)
Task #7 end

Task #8 start
Description: Global Secure ToolKit
Estimated time 30 second(s)
Task #8 end

Task #9 start
Description: Java support
Estimated time 12 second(s)
Task #9 end

Task #10 start
Description: SQL procedures
Estimated time 3 second(s)
Task #10 end

Task #11 start
Description: ICU Utilities
Estimated time 60 second(s)
Task #11 end

Task #12 start
Description: Java Common files
Estimated time 18 second(s)
Task #12 end

Task #13 start
Description: Base server support
Estimated time 318 second(s)
Task #13 end

Task #14 start
Description: Communication support - TCP/IP
Estimated time 3 second(s)
Task #14 end

Task #15 start
Description: EnterpriseDB code
Estimated time 4 second(s)
Task #15 end

Task #16 start
Description: Command Line Processor Plus
Estimated time 6 second(s)
Task #16 end

Task #17 start
Description: DB2 data source support
Estimated time 6 second(s)
Task #17 end

Task #18 start
Description: Spatial Extender server support
Estimated time 15 second(s)
Task #18 end

Task #19 start
Description: DB2 LDAP support
Estimated time 3 second(s)
Task #19 end

Task #20 start
Description: DB2 Instance Setup wizard
Estimated time 19 second(s)
Task #20 end

Task #21 start
Description: Base application development tools
Estimated time 34 second(s)
Task #21 end

Task #22 start
Description: DB2 Update Service
Estimated time 4 second(s)
Task #22 end

Task #23 start
Description: First Steps
Estimated time 3 second(s)
Task #23 end

Task #24 start
Description: Product Signature for DB2 Express-C
Estimated time 3 second(s)
Task #24 end

Task #25 start
Description: Sample database source
Estimated time 4 second(s)
Task #25 end

Task #26 start
Description: DB2 Text Search
Estimated time 92 second(s)
Task #26 end

Task #27 start
Description: Setting DB2 library path
Estimated time 180 second(s)
Task #27 end

Task #28 start
Description: Executing control tasks
Estimated time 20 second(s)
Task #28 end

Task #29 start
Description: Updating global registry
Estimated time 20 second(s)
Task #29 end

Task #30 start
Description: Starting DB2 Fault Monitor
Estimated time 10 second(s)
Task #30 end

Task #31 start
Description: Updating the db2ls link
Estimated time 1 second(s)
Task #31 end

Task #32 start
Description: Registering DB2 licenses
Estimated time 5 second(s)
Task #32 end

Task #33 start
Description: Setting default global profile registry variables
Estimated time 1 second(s)
Task #33 end

Task #34 start
Description: Initializing instance list
Estimated time 5 second(s)
Task #34 end

Task #35 start
Description: Registering DB2 Update Service
Estimated time 30 second(s)
Task #35 end

Task #36 start
Description: Updating global profile registry
Estimated time 3 second(s)
Task #36 end

The execution completed successfully.

For more information see the DB2 installation log at
"/tmp/db2_install.log.8332".
root@lodge:/tmp/expc# groupadd db2iadm1
root@lodge:/tmp/expc# groupadd db2fadm1
root@lodge:/tmp/expc# groupadd dasadm1
root@lodge:/tmp/expc# useradd -m -d /home/db2inst1 -g db2iadm1 db2inst1
root@lodge:/tmp/expc# useradd -m -d /home/db2fenc1 -g db2fadm1 db2fenc1
root@lodge:/tmp/expc# useradd -m -d /home/dasusr1 -g dasadm1 dasusr1
root@lodge:/tmp/expc# cd /opt/ibm/db2/V10.1/instance
root@lodge:/opt/ibm/db2/V10.1/instance# ls
common dasupdt db2dbmchk db2icrt_local db2iexec db2ipcld db2iupdt db2iver native
dascrt dasutil db2iauto db2idbm db2iinfo db2iset db2iupdt_local db2prechk sdSharedFileList
dasdrop db2chkf db2icknm db2idefs db2ilist db2isetup db2iupgrade db2ssh
daslist db2ckupgrade db2ickts db2idrop db2imchk db2isrv db2iupgrade_local db2uit
dasmigr db2clpid db2icrt db2idrop_local db2instcfg db2iuadm db2iutil disp_msg
root@lodge:/opt/ibm/db2/V10.1/instance# ./db2icrt -a server -p 51000 -u db2fenc1 db2inst1
DBI1446I The db2icrt command is running, please wait.


DB2 installation is being initialized.

Total number of tasks to be performed: 4
Total estimated time for all tasks to be performed: 309 second(s)

Task #1 start
Description: Setting default global profile registry variables
Estimated time 1 second(s)
Task #1 end

Task #2 start
Description: Initializing instance list
Estimated time 5 second(s)
Task #2 end

Task #3 start
Description: Configuring DB2 instances
Estimated time 300 second(s)
Task #3 end

Task #4 start
Description: Updating global profile registry
Estimated time 3 second(s)
Task #4 end

The execution completed successfully.

For more information see the DB2 installation log at "/tmp/db2icrt.log.31173".
DBI1070I Program db2icrt completed successfully.


root@lodge:/opt/ibm/db2/V10.1/instance# ./dascrt -u dasusr1
DBI1070I Program dascrt completed successfully.


root@lodge:/opt/ibm/db2/V10.1/instance# su - dasusr1
$ db2admin start
SQL4409W The DB2 Administration Server is already active.
$
root@lodge:/opt/ibm/db2/V10.1/instance# su - db2inst1
$ db2start
SQL1026N The database manager is already active.
$ db2_local_ps
-su: db2_local_ps: not found
$ bash
db2inst1@lodge:~$ db2_local_ps
bash: /home/db2inst1/sqllib/bin/db2_local_ps: /bin/ksh: bad interpreter: No such file or directory
db2inst1@lodge:~$ exit
$
root@lodge:/opt/ibm/db2/V10.1/instance# vi /etc/passwd
root@lodge:/opt/ibm/db2/V10.1/instance# su - db2inst1
$ db2stop
SQL1064N DB2STOP processing was successful.
$ db2start
SQL1063N DB2START processing was successful.
$ db2sampl

Creating database "SAMPLE"...
Connecting to database "SAMPLE"...
Creating tables and data in schema "DB2INST1"...
Creating tables with XML columns and XML data in schema "DB2INST1"...

'db2sampl' processing complete.

$ cat /etc/debian-version
cat: /etc/debian-version: No such file or directory
$ cat /etc/debian_version
6.0.5
$