Maximum number of WAL files in the pg_xlog directory (1)

时间：2023-01-22 21:41:47
 

Hi,

As part of our monitoring work for our customers, we stumbled upon an issue with our customers' servers who have a wal_keep_segments setting higher than 0.

We have a monitoring script that checks the number of WAL files in the pg_xlog directory, according to the setting of three parameters (checkpoint_completion_target, checkpoint_segments, and wal_keep_segments). We usually add a percentage to the usual formula:

greatest(
  (2 + checkpoint_completion_target) * checkpoint_segments + 1,
  checkpoint_segments + wal_keep_segments + 1
)

And we have lots of alerts from the script for customers who set their wal_keep_segments setting higher than 0.

So we started to question this sentence of the documentation:

There will always be at least one WAL segment file, and will normally not be more than (2 + checkpoint_completion_target) * checkpoint_segments + 1 or checkpoint_segments + wal_keep_segments + 1 files.

(http://www.postgresql.org/docs/9.3/static/wal-configuration.html)

While doing some tests, it appears it would be more something like:

wal_keep_segments + (2 + checkpoint_completion_target) * checkpoint_segments + 1

But after reading the source code (src/backend/access/transam/xlog.c), the right formula seems to be:

wal_keep_segments + 2 * checkpoint_segments + 1

Here is how we went to this formula...

CreateCheckPoint(..) is responsible, among other things, for deleting and recycling old WAL files. From src/backend/access/transam/xlog.c, master branch, line 8363:

/*

 * Delete old log files (those no longer needed even for previous

 * checkpoint or the standbys in XLOG streaming).

 */

if (_logSegNo)

{

    KeepLogSeg(recptr, &_logSegNo);

    _logSegNo--;

    RemoveOldXlogFiles(_logSegNo, recptr);

}

KeepLogSeg(...) function takes care of wal_keep_segments. From src/backend/access/transam/xlog.c, master branch, line 8792:

/* compute limit for wal_keep_segments first */

if (wal_keep_segments > )

{

    /* avoid underflow, don't go below 1 */

    if (segno <= wal_keep_segments)

        segno = ;

    else

        segno = segno - wal_keep_segments;

}

IOW, the segment number (segno) is decremented according to the setting of wal_keep_segments. segno is then sent back to CreateCheckPoint(...) via _logSegNo. The RemoveOldXlogFiles() gets this segment number so that it can remove or recycle all files before this segment number. This function gets the number of WAL files to recycle with the XLOGfileslop constant, which is defined as:

/*

 * XLOGfileslop is the maximum number of preallocated future XLOG segments.

 * When we are done with an old XLOG segment file, we will recycle it as a

 * future XLOG segment as long as there aren't already XLOGfileslop future

 * segments; else we'll delete it.  This could be made a separate GUC

 * variable, but at present I think it's sufficient to hardwire it as

 * 2*CheckPointSegments+1.  Under normal conditions, a checkpoint will free

 * no more than 2*CheckPointSegments log segments, and we want to recycle all

 * of them; the +1 allows boundary cases to happen without wasting a

 * delete/create-segment cycle.

 */

#define XLOGfileslop    (2*CheckPointSegments + 1)

(in src/backend/access/transam/xlog.c, master branch, line 100)

IOW, PostgreSQL will keep wal_keep_segments WAL files before the current WAL file, and then there may be 2*CheckPointSegments + 1 recycled ones. Hence the formula:

wal_keep_segments + 2 * checkpoint_segments + 1

And this is what we usually find in our customers' servers. We may find more WAL files, depending on the write activity of the cluster, but in average, we get this number of WAL files.

AFAICT, the documentation is wrong about the usual number of WAL files in the pg_xlog directory. But I may be wrong, in which case, the documentation isn't clear enough for me, and should be fixed so that others can't misinterpret it like I may have done.

Any comments? did I miss something, or should we fix the documentation?

Thanks.

Bruce Momjian:

I looked into this, and came up with more questions.  Why is 
checkpoint_completion_target involved in the total number of WAL 
segments?  If checkpoint_completion_target is 0.5 (the default), the 
calculation is:

(2 + 0.5) * checkpoint_segments + 1

while if it is 0.9, it is:

(2 + 0.9) * checkpoint_segments + 1

Is this trying to estimate how many WAL files are going to be created 
during the checkpoint?  If so, wouldn't it be (1 + 
checkpoint_completion_target), not "2 +".  My logic is you have the old 
WAL files being checkpointed (that's the "1"), plus you have new WAL 
files being created during the checkpoint, which would be 
checkpoint_completion_target * checkpoint_segments, plus one for the 
current WAL file.

The original calculation is summarized in this email:

http://www.postgresql.org/message-id/AANLkTi=e=oR54OuxAw88=dtV4wt0e5edMiGaeZtBVcKO@...

However, in my reading of this, it appears to be double-counting the WAL 
files during the checkpoint, e.g. the checkpoint_completion_target * 
checkpoint_segments WAL files are also part of the later 
checkpoint_segments number.

I also don't see how that can be equivalent to:

checkpoint_segments + wal_keep_segments + 1

because wal_keep_segments isn't used in the first calculation.  Is the 
user supposed to compute the maximum of those two?  Seems easier to just 
give one expression.

Is the right answer:

max(checkpoint_segments, wal_keep_segments) + checkpoint_segments + 1

or, if you want to use checkpoint_completion_target, it would be:

max(checkpoint_segments * checkpoint_completion_target, wal_keep_segments) + checkpoint_segments + 1

Is checkpoint_completion_target accurate enough to define a maximum 
number of files?

I think I need Masao Fujii's comments on this.  The fact the user is 
seeing something different from what is documented means something 
probably needs updating.

Jeff Janes:

I looked into this, and came up with more questions.  Why is
checkpoint_completion_target involved in the total number of WAL
segments?  If checkpoint_completion_target is 0.5 (the default), the
calculation is:

(2 + 0.5) * checkpoint_segments + 1

while if it is 0.9, it is:

(2 + 0.9) * checkpoint_segments + 1

Is this trying to estimate how many WAL files are going to be created
during the checkpoint?  If so, wouldn't it be (1 +
checkpoint_completion_target), not "2 +".  My logic is you have the old
WAL files being checkpointed (that's the "1"), plus you have new WAL
files being created during the checkpoint, which would be
checkpoint_completion_target * checkpoint_segments, plus one for the
current WAL file.

 
WAL is not eligible to be recycled until there have been 2 successful checkpoints.
 
So at the end of a checkpoint, you have 1 cycle of WAL which has just become eligible for recycling,
1 cycle of WAL which is now expendable but which is kept anyway, and checkpoint_completion_target worth of WAL which has occurred while the checkpoint was occurring and is still needed for crash recovery.
 
I don't really understand the point of this way of doing things.  I guess it is because the control file contains two redo pointers, one for the last checkpoint, and one for the previous to that checkpoint, and if recovery finds that it can't use the most recent one it tries the ones before that.  Why?  Beats me.  If we are worried about the control file getting a corrupt redo pointer, it seems like we would record the last one twice, rather than recording two different ones once each.  And if the in-memory version got corrupted before being written to the file, I really doubt anything is going to save your bacon at that point.
 
I've never seen a case where recovery couldn't use the last recorded good checkpoint, so instead used the previous one, and was successful at it.  But then again I haven't seen all possible crashes.
 
This is based on memory from the last time I looked into this, I haven't re-verified it so could be wrong or obsolete.
 
 
 
> WAL is not eligible to be recycled until there have been 2 successful 
> checkpoints. 
> 
> So at the end of a checkpoint, you have 1 cycle of WAL which has just become 
> eligible for recycling, 
> 1 cycle of WAL which is now expendable but which is kept anyway, and 
> checkpoint_completion_target worth of WAL which has occurred while the 
> checkpoint was occurring and is still needed for crash recovery.
«  []

OK, so based on this analysis, what is the right calculation?  This?

(1 + checkpoint_completion_target) * checkpoint_segments + 1 + 
        max(wal_keep_segments, checkpoint_segments) 

 
 
 
> AFAICT, the documentation is wrong about the usual number of WAL files in 
> the pg_xlog directory. But I may be wrong, in which case, the documentation 
> isn't clear enough for me, and should be fixed so that others can't 
> misinterpret it like I may have done. 
> 
> Any comments? did I miss something, or should we fix the documentation?
«  []

I think you're right. The correct formula of the number of WAL files in 
pg_xlog seems to be

(3 + checkpoint_completion_target) * checkpoint_segments + 1

or

wal_keep_segments + 2 * checkpoint_segments + 1

Why? At the end of checkpoint, the WAL files which were generated since the 
start of previous checkpoint cannot be removed and must remain in pg_xlog. 
The number of them is

(1 + checkpoint_completion_target) * checkpoint_segments

or

wal_keep_segments

Also, at the end of checkpoint, as you pointed out, if there are 
*many* enough old WAL files, 2 * checkpoint_segments + 1 WAL files will be 
recycled. Then checkpoint_segments WAL files will be consumed till the end of 
next checkpoint. But since there are already 2 * checkpoint_segments + 1 
recycled WAL files, no more files are increased. So, WAL files that we cannot 
remove and can recycle at the end of checkpoint can exist in pg_xlog, and the 
num of them can be calculated by the above formula.

If my understanding is right, we need to change the formula at the document.

 
 
 
 
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