VACUUM hanging on idle system

I have a completely idle postgresql system (all backends "idle", none
"in transaction"); every time I attempt to vacuum a particular table,
it hangs
after a while. Here is the output of vacuumdb:

INFO:  vacuuming "public.ledgerdetail"
INFO:  index "ledgerdetail_pkey" now contains 11574842 row versions in
33032
pages
DETAIL:  1 index row versions were removed.
42 index pages have been deleted, 42 are currently reusable.
CPU 1.23s/1.57u sec elapsed 12.13 sec.

The table has another index besides that one. In the system error log
there
is this line:
   could not write block 32756 of relation 1663/134097/385649401:
   No space left on device
The referenced block is in the table's index file. The disk has plenty
of space.
I've seen that semaphore operations can give an "out of space" error. I
wrote
a program to dump the state of the semaphores, and ran it with the
system
idle and with the vacuum hanging. There are about 25 semaphore sets in
the
system, but only one changes. Here is the before and after:

    0     1     2     3     4     5     6     7     8     9    10    11
   12    13    14    15    16
pid  ...
value ...
ncnt  ...
zcnt  ...

Set 131091
47542 47395 47376 47375 47374 47373 47372 47371 84426 47370 47541 45292
46849 46810 46809 46808   530
    0     0     0     0     0     0     0     0     0     0     0     0
    0     0     0     0   537
    0     0     0     0     0     0     0     0     0     0     0     0
    0     0     0     0     0
    0     0     0     0     0     0     0     0     0     0     0     0
    0     0     0     0     0

Set 131091
47542 47395 47376 47375 47374 47373 47372 47371 84426 47370 47541 45381
46849 46810 46809 46808   530
    0     0     0     0     0     0     0     0     0     0     0     0
    0     0     0     0   537
    0     0     0     0     0     0     0     0     0     0     0     1
    0     0     0     0     0
    0     0     0     0     0     0     0     0     0     0     0     0
    0     0     0     0     0

The pid of the backend doing the vacuum is 45381, and it seems to be
waiting
on semaphore #11. (I have the complete dump if it's of interest.)

Someone tried to drop the index and got this:
ERROR:  FlushRelationBuffers("idx_ld1", 0): block 32756 is referenced
(private
0, global 1)

I'm going to restart postgresql and see if any possible semaphore
problem
goes away.


Any ideas about what the problem is here?

Thanks.