Re: AW: Size of /pgdata

On 6/18/26 07:59, William Sescu (Suva) wrote:
> By setting XFS allocsize, you switch off the dynamic behavior, and it
> becomes more predictable.
>
> Another fix might be a switch to a different filesystem. E.g. ext4
>
> While this XFS feature is a quite old one, I have seen it the first time
> kicking in quite heavily only on Red Hat 9, with PostgreSQL 18 and
>
> only on the standby. Some times double the size for some files.
>
> *Von:*Raj <rajeshkumar(dot)dba09(at)gmail(dot)com>
> *Gesendet:* Mittwoch, 17. Juni 2026 20:46
> *An:* Sescu William (SW0) <william(dot)sescu(at)suva(dot)ch>
> *Cc:* Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>; Pgsql-admin <pgsql-
> admin(at)lists(dot)postgresql(dot)org>
> *Betreff:* 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
> <mailto: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 <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
> <mailto:laurenz(dot)albe(at)cybertec(dot)at>>
> Gesendet: Mittwoch, 17. Juni 2026 13:34
> An: Raj <rajeshkumar(dot)dba09(at)gmail(dot)com
> <mailto:rajeshkumar(dot)dba09(at)gmail(dot)com>>
> Cc: Pgsql-admin <pgsql-admin(at)lists(dot)postgresql(dot)org <mailto: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
>
>
>
> ________________________________
>
> Disclaimer:
>
> Diese Nachricht und ihr eventuell angehängte Dateien sind nur für
> den Adressaten bestimmt. Sie kann vertrauliche oder gesetzlich
> geschützte Daten oder Informationen beinhalten. Falls Sie diese
> Nachricht irrtümlich erreicht hat, bitten wir Sie höflich, diese
> unter Ausschluss jeglicher Reproduktion zu löschen und die
> absendende Person zu benachrichtigen. Danke für Ihre Hilfe.
>
> This message and any attached files are for the sole use of the
> recipient named above. It may contain confidential or legally
> protected data or information. If you have received this message in
> error, please delete it without making any copies whatsoever and
> notify the sender. Thank you for your assistance.
>

R U joking? Preallocation is around kilobytes, some time megabytes.