Date: Thu, 25 Mar 2004 05:22:00 -0800
From: Chris Wedgwood <cw@f00f.org>
To: Stefan Smietanowski <stesmi@stesmi.com>
Cc: "IKARASHI, Seiichi" <ikarashi@miraclelinux.com>, linux-xfs@oss.sgi.com
Subject: Re: synchronization of XFS
Message-ID: <20040325132200.GA12333@dingdong.cryptoapps.com>
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On Thu, Mar 25, 2004 at 02:00:21PM +0100, Stefan Smietanowski wrote:

> Grub does it as a workaround.

It's stupid.  How does sync() multiple times in secession work better
than once?

> Happens with current tree I can tell you that me and .. Steve I
> think discussed it on IRC a while back and he looked into the POSIX
> spec and according to POSIX a filesystem does not have to keep the
> block device the same as the filesystem after a sync() (or mounting
> SYNC).

Well, not to split hair, sync() means all data needs to hit the disk.
Accessing it via a bock device for a mounted filesystem will cause bad
stuff --- is this what grub does?

I umount/mount should enforce everything is written an consistent.
Grub could also use O_DIRECT if it didn't trust direct-block access
was going to be in sync. with the fs after a sync() call (which as you
say strictly speaking it need not be).

> And Grub relies on the filesystem keeping the block device in sync
> with the filesystem after a sync() call. That's a bug in both Grub
> and XFS but XFS isn't the one violating the spec, even though it's a
> bug.

If the filesystem is mounted, grub is doing something it can't rely
on.  Grub could use O_DIRECT probably to work around this.