/var/lib/pgsql/data/pg_xlog/000000010000000000000000,two process access it ?

I guess PID 19045 write data to the log file first when I insert data
into table ,but why did writer process also access the log file ?
Could some guy tell me some details ?
FYI:
postgres: writer process's PID is 18848 .
postgres test [local] idle's PID is 19045  .

[root@localhost tmp]# lsof /var/lib/pgsql/data/pg_xlog/
000000010000000000000000
COMMAND     PID     USER   FD   TYPE DEVICE     SIZE    NODE NAME
postmaste 18848 postgres    4u   REG  253,0 16777216 1770912 /var/lib/
pgsql/data/pg_xlog/000000010000000000000000
postmaste 19045 postgres   37u   REG  253,0 16777216 1770912 /var/lib/
pgsql/data/pg_xlog/000000010000000000000000


[root@localhost tmp]# ps aux|grep postgre
postgres  2429  0.0  0.6  21044  3364 ?        S    07:32   0:00 /usr/
bin/postmaster -p 5432 -D /var/lib/pgsql/data
postgres  2434  0.0  0.1  10824   804 ?        S    07:32   0:00
postgres: logger process
root      9539  0.0  0.2   4908  1232 pts/3    S    15:31   0:00 su -
postgres
postgres  9540  0.0  0.2   4528  1480 pts/3    S    15:31   0:00 -bash
postgres 18848  0.0  0.2  21180  1324 ?        S    17:23   0:00
postgres: writer process
postgres 18849  0.0  0.1  11824   780 ?        S    17:23   0:00
postgres: stats buffer process
postgres 18850  0.0  0.1  11056  1000 ?        S    17:23   0:00
postgres: stats collector process
postgres 19029  0.0  0.4   8292  2296 pts/3    S+   17:25   0:00 psql
postgres 19045  0.0  0.7  21888  4016 ?        S    17:25   0:00
postgres: postgres test [local] idle
root     19607  0.0  0.1   3912   696 pts/2    R+   17:31   0:00 grep
postgre