Question about Restart point and checkpoint_segments - Mailing list pgsql-hackers

<div dir="ltr">Hello,<br /><br />I am using a hot_standby setup on PostgreSQL 9.1<br />While I was testing, I found out
thatonly checkpoint_timeout (+ a checkpoint since the last restart point) could trigger a restart point. <br /><br
/>Thecode (bgwriter.c) seems to confirm this:<br /><br />    /*<br />    * Check progress against WAL segments written
andcheckpoint_segments.<br />    *<br />    * We compare the current WAL insert location against the location<br />   
*computed before calling CreateCheckPoint. The code in XLogInsert that<br />    * actually triggers a checkpoint when
checkpoint_segmentsis exceeded<br />    * compares against RedoRecptr, so this is not completely accurate.<br />    *
However,it's good enough for our purposes, we're only calculating an<br />    * estimate anyway.<br />    */<br />   
if(!RecoveryInProgress())  ===> Only in case of primary<br />    {<br />                   recptr =
GetInsertRecPtr();<br/>                   elapsed_xlogs =<br />                                   (((double) (int32)
(recptr.xlogid- ckpt_start_recptr.xlogid)) * XLogSegsPerFile +<br />                                   ((double)
recptr.xrecoff- (double) ckpt_start_recptr.xrecoff) / XLogSegSize) /<br />                                  
CheckPointSegments;<br/><br />                   if (progress < elapsed_xlogs)  ===> progress in volume<br />   
              {<br />                                   ckpt_cached_elapsed = elapsed_xlogs;<br />                   
              return false;<br />                   }<br />    }<br /><br />    /*<br />    * Check progress against
timeelapsed and checkpoint_timeout.<br />    */<br />    gettimeofday(&now, NULL);<br />    elapsed_time =
((double)((pg_time_t) now.tv_sec - ckpt_start_time) +<br />                                                           
     now.tv_usec / 1000000.0) / CheckPointTimeout;<br /><br /><br />    if (progress < elapsed_time)  ===>
progressin time<br />    {<br />                   ckpt_cached_elapsed = elapsed_time;<br />                   return
false;<br/>    }<br /><br />    /* It looks like we're on schedule. */<br />    return true;<br />                <br
/>Ialso found a post from Simon Riggs [1]: "checkpoint_segments is ignored on standby."<br /><br />The documentation is
statingthe opposite [2]: "In standby mode, a restartpoint is also triggered if checkpoint_segments log segments have
beenreplayed since last restartpoint and at least one checkpoint record has been replayed."<br /><br />Since I am not a
nativeenglish speaker, maybe I misunderstood the documentation. But to me, it looks wrong.<br />If it's indeed wrong.
Couldyou explain why checkpoint_segments doesn't trigger a restart_point in standby mode ?<br /><br />Thank you<br
/>Benoit<br/><br />[1] <a
href="http://www.postgresql.org/message-id/CA+U5nMKdf7odZzYNnoRkkCZmJpGEy=OQbU9Nan_zva_Rtzi2vw@mail.gmail.com">http://www.postgresql.org/message-id/CA+U5nMKdf7odZzYNnoRkkCZmJpGEy=OQbU9Nan_zva_Rtzi2vw@mail.gmail.com</a><br
/>[2]<a
href="http://www.postgresql.org/docs/9.1/static/wal-configuration.html">http://www.postgresql.org/docs/9.1/static/wal-configuration.html</a><br
/></div>