Re: Size of /pgdata

Yes, this is what I have been asking. I also mentioned/asked earlier
whether it's Abt XFS file system.

I checked bitmaps and under flags I could find 'unwritten preallocated
extent'. --apparent-size says shows correct size.

Is the fix - setting up allocsize?

On Wed, 17 Jun 2026, 19:29 William Sescu (Suva), <william(dot)sescu(at)suva(dot)ch>
wrote:

> In case you are using XFS, this might be related to the "Speculative
> preallocation" feature.
> We saw the same behavior on Redhat 9, with PostgreSQL 18 and XFS.
>
>
> https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/storage_administration_guide/migrating-ext4-xfs
>
> Speculative preallocation
>
> XFS uses speculative preallocation to allocate blocks past EOF as files
> are written. This avoids file fragmentation due to concurrent streaming
> write workloads on NFS servers. By default, this preallocation increases
> with the size of the file and will be apparent in "du" output. If a file
> with speculative preallocation is not dirtied for five minutes the
> preallocation will be discarded. If the inode is cycled out of cache before
> that time, then the preallocation will be discarded when the inode is
> reclaimed.
> If premature ENOSPC problems are seen due to speculative preallocation, a
> fixed preallocation amount may be specified with the -o allocsize=amount
> mount option.
>
> See also "man xfs"
>
> allocsize=size
> Sets the buffered I/O end-of-file preallocation size when doing delayed
> allocation writeout. Valid values for this option are page size (typically
> 4KiB) through to
> 1GiB, inclusive, in power-of-2 increments.
>
> The default behavior is for dynamic end-of-file preallocation size, which
> uses a set of heuristics to optimise the preallocation size based on the
> current allocation
> patterns within the file and the access patterns to the file. Specifying a
> fixed allocsize value turns off the dynamic behavior.
>
>
> Cheers
> William
>
>
> -----Ursprüngliche Nachricht-----
> Von: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
> Gesendet: Mittwoch, 17. Juni 2026 13:34
> An: Raj <rajeshkumar(dot)dba09(at)gmail(dot)com>
> Cc: Pgsql-admin <pgsql-admin(at)lists(dot)postgresql(dot)org>
> Betreff: Re: Size of /pgdata
>
>
>
> ACHTUNG: Diese Nachricht kommt von extern. Seien Sie kritisch beim
> Öffnen von Links und Anhängen.
>
>
>
>
> On Wed, 2026-06-17 at 11:47 +0530, Raj wrote:
> > The file count is same between them..but the file size varies.
> >
> > For example, assume node1 has issue and node2 is clean.
> >
> > 1. On both nodes file count is 76
> > 2. File size varies for some of them
> >
> > Each files are either 1GB or 2GB and some or them are 1.2gb or 1.5gb or
> between 1-2gb ....by looking at files..
> > In the problematic node, some of the 1gb files in node2 shows 2GB in
> node1...and also file size differences. making the difference.
>
> That is highly suspicious.
>
> What do you get if you run
>
> SHOW segment_size;
>
> Unless you built PostgreSQL yourself after changing the segment size, it
> should be 1GB. But then it would be impossible for some segment to be
> bigger than 1GB.
>
> Are you using some weird file system that reports file sizes wrongly?
>
> What does "ls -l" on one of these big files show?
>
> Yours,
> Laurenz Albe
>
>
>
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