Configuration of pm_buffer behind om_file

Hi,

Here's my case :

I'm trying to create Syslog Server Configuration, based on CentOS. Server must :
-- Receive log from tcp
-- store log into a NFS Volume automaticaly mounted by fstab on server startup
---- in case of fail of the NFS Volume, create a buffer on local disk.

Here's my configuration :

<Extension _syslog>
Module xm_syslog
</Extension>

<Extension _xm_file>
Module xm_fileop
</Extension>

<Input in>
Module im_tcp
Host 0.0.0.0
Port 39458
<Exec>
$SERVER = hostname();
parse_syslog_ietf();
$DATE = strftime($EventTime, "%Y%m%d");
</Exec>
</Input>

<Processor buffer>
Module pm_buffer
maxSize 204800
Type Disk
Directory %LOGDIR%
WarnLimit 190000
</Processor>

<Processor blocker>
Module pm_blocker
<Exec>
$CHEMIN = "/var/partage/"+$SERVER;
$test = dir_exists($CHEMIN);
if $test == TRUE
{
blocker->block(FALSE);
}
else blocker->block(TRUE);
</Exec>
</Processor>

<Output out>
Module om_file
File "/var/partage/"+$SERVER+"/"+$Hostname+"/"+$DATE+"_"+$Hostname+".log"
CreateDir TRUE
Exec to_syslog_ietf();
</Output>

<Route tcp_to_file>
Path in => buffer => blocker => out
</Route>

My Problem :
- When NFS Volume is down, pm_blocker write " can't check condition "dir_exists(<Path>)" : input/outpur Error"
- When i try without pm_blocker, om_file fails with same error and no buffer created....

Question :
- What is the best solution to implement a disk buffer system for om_file module ?
- How can i catch NFS volume exceptions ?

Thanks in advance,
best regards,
Julien