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Re: data corruption/loss

To: Andi Kleen <ak@xxxxxxx>
Subject: Re: data corruption/loss
From: Christian Zander <phoenix@xxxxxxxxx>
Date: Sun, 18 Nov 2001 19:37:38 +0100
Cc: Christian Zander <phoenix@xxxxxxxxx>, linux-xfs@xxxxxxxxxxx
In-reply-to: <20011118095940.A6147@xxxxxxxxxxxxx>
References: <20011118090353.A17758@chronos> <20011118095940.A6147@xxxxxxxxxxxxx>
Reply-to: Christian Zander <phoenix@xxxxxxxxx>
Sender: owner-linux-xfs@xxxxxxxxxxx
User-agent: Mutt/
On Sun, Nov 18, 2001 at 09:59:40AM +0100, Andi Kleen wrote:
> You're really asking a FAQ. Please check the FAQ.

I did scan through it several times; I guess I wasn't looking for
the right question. My apologies for that, I will read through it
again as soon as oss.sgi.com comes back up.

> In short what you're seeing is that the files have their metadata
> (file size) but not their data flushed yet. The file has turned
> into an hole. One of the reasons XFS is so fast is that it delays
> data flushing until it has enough data to allocate a big chunk on
> disk. Because of that it can happen that the data has not made it
> to disk yet, but the file size has, turning the file into a hole
> (= zeroes). The journaling file system does only protect the
> metadata, not the data.

Ok, that makes sense. I'm not overly familiar with file system
concepts, but of what use is metadata if the corresponding data
never makes it to the disk?

> Possibilities are to type sync manually, change your editor to use
> fsync/O_SYNC/O_DSYNC to force data to disk or mount the filesystem
> with one of the slower-but-somewhat safer sync/dsync etc. options.

Changing all applications really isn't an option, but I will take a
look at the mount options.

> The other file systems have the same problem BTW, although it is
> less often seen in practice because they don't delay writes as
> aggressively as XFS.

>From your explanation I can imagine that it happens on other file
systems as well. I've been relying on journaling file systems for
quite a while now, however, and have never experienced anything
alike with anything other than xfs.

Well, either way, I appreciate your quick response despite my
failing to 'read the fscking documentation' carefully enough.

christian zander

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