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Re: Question related to XFS sync , especially fsync

To: "Christoph Hellwig" <hch@xxxxxxxxxxxxx>
Subject: Re: Question related to XFS sync , especially fsync
From: "Gopala Krishna" <gopalakrishna.n.m@xxxxxxxxx>
Date: Wed, 16 Jan 2008 14:30:14 +0530
Cc: "Chris Wedgwood" <cw@xxxxxxxx>, nscott@xxxxxxxxxx, xfs@xxxxxxxxxxx
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Thanks for the suggestions!.  I will relook at the my idea and
design....implemetation. As of now what we are doing is in experimental
stage.

Thanks,
Gopal.


On 1/16/08, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 16, 2008 at 12:55:17PM +0530, Gopala Krishna wrote:
> > While  replying  to Eric, I mentioned why we are doing that. We are
> > basically providing interfaces to back up applications in a pure storage
> > environment that deals with the  back up at block level (sector level)
> and
> > hence depending upon different file system, we need to get information
> about
> > file like it's extent information and associated block numbers etc.
>
> This basically can't work.  If you do a plain block based backup you
> need to freeze the filesystem first and then either backup through a
> newly created snapshot or the raw device.  Alternatively you can do
> file-based backups assisted by the bulkstat interface as done by
> xfsdump.  If you try to mix the two layers you get into deep trouble
> due to various issues:
>
> - knowledge of the disk format.  The ondisk format can change anytime
>   and will break your application.  And yes, additions to the ondisk
>   format do happen quite frequently.
> - no coherency between the filesystem and the block device node.  This
>   is especially true for backup applications which use the buffered
>   block device nodes because there is a real-life chance that stale
>   cache is around
> - no guarantee that the ondisk image is actually update.  XFS like
>   most other current filesystems uses an intent log to provide
>   reliabily and sync is only guaranteed to push updates into the log
>   but not actually write it back to it's "normal" location on disk.
>
> In short what you're trying to do is a road to disaster, so don't do it!
>
> Note that the problems apply to any filesystem in one way or another,
> not just XFS.
>
>


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