Date: Tue, 15 Aug 2006 16:58:59 -0400
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: Postgresql Performance on an HP DL385 and
In-reply-to: <20060815193951.GA13695@mark.mielke.cc>
To: mark@mark.mielke.cc
Cc: pgsql-performance@postgresql.org
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On Tue, Aug 15, 2006 at 03:39:51PM -0400, mark@mark.mielke.cc wrote:
>No. This is not true. Updating the file system structure (inodes, indirect
>blocks) touches a separate part of the disk than the actual data. If
>the file system structure is modified, say, to extend a file to allow
>it to contain more data, but the data itself is not written, then upon
>a restore, with a system such as ext2, or ext3 with writeback, or xfs,
>it is possible that the end of the file, even the postgres log file,
>will contain a random block of data from the disk. If this random block
>of data happens to look like a valid xlog block, it may be played back,
>and the database corrupted.

you're conflating a whole lot of different issues here. You're ignoring 
the fact that postgres preallocates the xlog segment, you're ignoring 
the fact that you can sync a directory entry, you're ignoring the fact 
that syncing some metadata (such as atime) doesn't matter (only the 
block allocation is important in this case, and the blocks are 
pre-allocated).

>This is also wrong. fsck is needed because the file system is broken.

nope, the file system *may* be broken. the dirty flag simply indicates 
that the filesystem needs to be checked to find out whether or not it is 
broken.

>I don't mean to be offensive, but I won't accept what you say, as it does
>not make sense with my understanding of how file systems work. :-)

<shrug> I'm not getting paid to convince you of anything.

Mike Stone