Re: PostgreSQL occasionally unable to rename WAL files (NTFS)

From: Thorsten Schöning <tschoening(at)am-soft(dot)de>
To: <pgsql-general(at)postgresql(dot)org>
Subject: Re: PostgreSQL occasionally unable to rename WAL files (NTFS)
Date: 2021-02-11 15:33:11
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Guten Tag Guy Burgess,
am Donnerstag, 11. Februar 2021 um 01:21 schrieben Sie:

> What appears to be happening is the affected WAL files (which is
> usually only 2 or 3 WAL files at a time) are somehow "losing" their
> NTFS permissions, so the PG process can't rename them - though of
> course the PG process created them. Even running icacls as admin
> gives "Access is denied" on those files. A further oddity is the
> affected files do end up disappearing after a while.

If you see that somewhat frequently, use Process Monitor and Process
Explorer to see who accesses those files how. ProcExp easily allows
you to find all open handles per file. If it's not AV, it might be
something like Windows Search Indexer as well, if that is enabled by
default in Server 2019 at all.

Though, even with my Windows 10 and Search Indexer enabled I didn't
run into such problems yet. And especially when rewriting large parts
of my databases with lots of created WAL files, I see the Indexer
working on those files, but NOT conflicting with Postgres yet.

The behaviour you describe happens exactly when two processes e.g.
concurrently hold HANDLEs on the same file and one of those deletes
the file then. Windows keeps file names until all open HANDLEs are
closed and depending on how those HANDLEs have been opened by the
first app, concurrent deletion is perferctly fine for Windows.

Though, a such deleted file can't be opened easily anymore and looks
like it has lost permissions only. But that's not the case, it's
deleted already. It might be that this happens for Postgres to itself
somehow when some other app has an open HANDLE. I don't think that
some other app is deleting that file by purpose instead, reading it
for some reason seems more likely to me.

> dwShareMode

> Enables subsequent open operations on a file or device to request
> delete access. Otherwise, other processes cannot open the file or
> device if they request delete access.

Mit freundlichen Grüßen

Thorsten Schöning

Thorsten Schöning
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Für Rückfragen stehe ich Ihnen sehr gerne zur Verfügung.

Mit freundlichen Grüßen

Thorsten Schöning

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