Manic supplied a quick command to manage the AIX wtmp file should it get too big for the system to handle.
last command will fail if the file is over 2Gb in size.
First convert the file to ASCII redirecting the output to a filesystem with enough space.
usr/lib/acct/fwtmp < /var/adm/wtmp > /tmp/wtmp.ascii.20110128
Now we compress it and clear out the original wtmp file.
compress /tmp/wtmp.ascii.20110128 >/var/adm/wtmp
If, however, you wish to just prune the file, in SunOS (for example) you can do…
/usr/lib/acct/fwtmp < /var/adm/wtmpx > /tmp/wtmpx.orig tail -20000 /tmp/wtmpx.orig > /tmp/wtmpx.new /usr/sbin/acct/fwtmp -ic < /tmp/wtmpx.new > /var/adm/wtmpx rm /tmp/wtmpx.orig /tmp/wmtpx.new
A similar approach can be used in AIX.