Re: configuring TSM with HACMP and informix : help needed

good question
that is good planning job :-)
It is not possible to answer just in few words, so I'd rather refer to docs.
You can find some good information on http://www.redbooks.ibm.com/ , for
example, look through:
Informix Cluster POWERsolution Guide, SG24-2020-00 Redbook, published
December-31-1997
SG244498 HACMP/ES Customization Examples.
There are some other good books.
That give you an idea.
I didn't understand your question: "Problem is as follows : what about a
restoration of my informix db back
to system A ?"  I guess you mix something if not sorry...HACMP assumes that
shared disks should be used so all shared filesystem can be accessed from
both nodes. In addition you can use cross-mounted NFS if you want to access
it at the same time. That means you do not need that restoration at all.
However you can restore files from A node backup to node B. To do that just
use dsmB.opt instead of dsmA.opt on node B or change client nodename
(virtialnode) option in corresponding dsm.opt / dsm.sys.

Just briefly about HACMP configuration in general: (IBM calls that as
"mutual takeover" configuration)
(there are many options). It's better to draw some picture but ...
I'd install the following on shared disks:
       (1)  - tsm server soft/config files, tsm stgpools, tsm  db/log,
       (you have two options: it can be installed on shared or local disks
(rootvg) as well )
       (2) informix db A (belongs to A node - service addr A, standby -B)
       (3) informix db B (belongs to B node - service addr B, standby - A)
       (4) informix soft ( ? the same as for tsm server soft)

By default tsm dirs and files are installed in /usr filesystem (local disk,
rootvg) but you can change it later to shared disks using mount facility. I
believe it works for informix as well.

and HACMP as an example:
Node A
service IP - A, standby IP - B
resource group:
   filesystem:
     informix db A (informix soft/config files A )
     tsm stgpool (? tsm server soft/config files)
   application servers:
     informix A
     tsm server
     tsm client A (can be configured using dsmA.opt )

Node B
service IP - B, standby IP - A
resource group:
   filesystems:
     informix db B (informix soft/config files B )
   appl servers:
     informix B
     tsm client B (can be configured using dsmB.opt )

Configuration assumes that you have tsm backup library as shared (Highly
available) resource (using SCSI differential interface connected to both
nodes). If not you can use two tsm servers but they should be out of HACMP.
sorry for my spelling
good luck