naming of wal-archives

Hi all,

i am not sure whether i have fully understood the implications of
archiving wal-files and i have a few questions.

We currently keep a rather long backlog of wal-files, since we have a few
hot-standbys over slow and unreliable lines that might fall back. So this
is an extract from my current pg_xlog directory:

-rw------- 1 postgres postgres 16777216 Jan 29 16:03
000000010000008C0000008E
-rw------- 1 postgres postgres 16777216 Jan 29 16:32
000000010000008C0000008F
-rw------- 1 postgres postgres 16777216 Jan 29 16:35
000000010000008C00000090
-rw------- 1 postgres postgres 16777216 Jan 4 15:02
000000010000008C00000091
-rw------- 1 postgres postgres 16777216 Jan 4 15:02
000000010000008C00000092

As you can see it recycles existing files by using them with their exact
name as they already exist (next file to be overwritten is the ..91-file).
So far so good. I have, just a few minutes ago, set archive_mode to "on"
and set an archive_command and restarted the server. Now files start
appearing in the wal-archive directory. But i seem to have two "lines" of
wal archives in my wal-storage directory, as the following extract shows:

-rw------- 1 postgres postgres 16777216 Jan 30 09:03
000000010000008D00000036
-rw------- 1 postgres postgres 16777216 Jan 30 09:05
000000010000008D00000037
-rw------- 1 postgres postgres 16777216 Jan 30 09:09
000000010000008D00000038
-rw------- 1 postgres postgres 16777216 Jan 30 09:16
000000010000008D00000039
...
-rw------- 1 postgres postgres 16777216 Jan 30 09:09
000000010000006D00000016
-rw------- 1 postgres postgres 16777216 Jan 30 09:09
000000010000006D00000017
-rw------- 1 postgres postgres 16777216 Jan 30 09:24
000000010000006D00000018
-rw------- 1 postgres postgres 16777216 Jan 30 09:24
000000010000006D00000019

These files are current. But one is the ..8D.. line and the other one is
the ..6D.. line and both lines are being added to. I also have another
server that connects to the master via pg_receivexlog and that one only
shows the ...8D.. line.

My questions are as follows:

Why is postgres reusing the old files with the old names ?

Is there a difference in the naming of wal-files if archive_mode = on
compared to archive_mode = off ?

Does archive_mode = on combined with the use of a proper archive-command
ensure that the naming of the wal-files will never collide with existing
files ? If not, how do i prevent files from being overwritten when using an
archive_command ? And how would i restore the needed file names for
recovery if i decide to keep one base-backup und then a very long chain of
wal-files ?

Is storing the output of pg_receivexlog equivalent to using archive_mode =
on and using a proper archive_command ?

I have spent a more than a few hours on this, but am currently somewhat
lost.

Thanks in advance,

Neil