Vacuum writes on empty system

I just loaded up 9GB of data (32 tables) into a brand new system (Postgres
7.4.5 on Linux 2.4 with 1GB ram, IDE). I immediately ran "vacuum analyze"
but Postgres seems to be writing a lot to disk even though none of the
tables have incurred any UPDATES or DELETES yet and there are no other
connections or process running. Is it normal for vacuum to be writing to
disk even when there is nothing to clean up?

procs memory swap io system
cpu
r b swpd free buff cache si so bi bo in cs us sy id
wa
0 2 0 18860 76336 870436 0 0 109 439 420 495 3 1 86
10
0 2 0 20076 76344 870480 0 0 640 84 510 171 0 0 0
100
0 2 0 19320 76344 871244 0 0 880 1636 392 244 0 0 0
100
0 1 0 18920 76348 871740 0 0 876 940 317 295 13 0 14
73
0 1 0 17184 76348 873476 0 0 1408 0 279 382 0 0 0
100
0 1 0 16244 76348 874416 0 0 1280 0 271 404 0 0 0
99
0 2 0 18484 76348 871088 0 0 1408 4560 323 415 0 1 0
99

avg-cpu: %user %nice %sys %idle
3.66 0.00 0.50 95.84

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s
avgrq-sz avgqu-sz await svctm %util
/dev/hdc 32.67 206.11 27.92 37.70 484.68 1951.29 242.34 975.65
37.12 2.93 15.50 16.93 111.13

db=# vacuum verbose analyze;
INFO: analyzing "public.table_3e"
INFO: "table_3e": 3233 pages, 3000 rows sampled, 69814 estimated total rows
INFO: vacuuming "public.table_3f"
INFO: index "table_3f_pkey" now contains 68928 row versions in 460 pages
DETAIL: 0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.13 sec.
INFO: "table_3f": found 0 removable, 68928 nonremovable row versions in
3255 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.06s/0.02u sec elapsed 18.71 sec.
INFO: vacuuming "pg_toast.pg_toast_1439762"
INFO: index "pg_toast_1439762_index" now contains 134585 row versions in
516 pages
DETAIL: 0 index pages have been deleted, 0 are currently reusable.
CPU 0.07s/0.02u sec elapsed 4.37 sec.
INFO: "pg_toast_1439762": found 0 removable, 134585 nonremovable row
versions in 29894 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 1.08s/0.24u sec elapsed 53.02 sec.
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