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Re: Changing root journal data mode

On Thu, Sep 06, 2001 at 10:06:45AM +0100, Stephen C. Tweedie wrote:
> On Wed, Sep 05, 2001 at 09:06:03PM -0700, Mike Fedyk wrote:
> > On Wed, Sep 05, 2001 at 09:57:27PM -0600, Andreas Dilger wrote:
> > > unmounted and mounted again.  Many of the superblock fields are only
> > > checked at mount time.
> > > 
> > 
> > Hmm, what about root reservations?  What keeps the kernel from overwriting
> > the changed superblocks with something from the page cache?
> > 
> > IIRC, tune2fs uses the buffer cache to make its changes, and they aren't
> > kept in sync...
> The kernel also uses the buffer cache when dealing with the
> superblock.  There are still some cache aliasing problems --- the
> kernel will read several superblock fields from the buffer cache and
> store them in the internal ext2/3 superblock struct at times, mainly
> to avoid constant byte-swapping when accessing those fields, but the
> fields where that is done tend to be mostly readonly so if tune2fs
> modifies a few bytes of the buffer-cache superblock, those values
> won't be overwritten by the kernel.  They won't be *read* by the
> kernel until next mount, but they will survive on disk.

Ok, great.  Was this true on 2.2 and 2.0 kernels also?

Were there any fields that the kernel didn't treat as read only?  I can
imagine that the FS_CLEAN flag would be written...

Also, I noticed that on 2.4.9-ac7 if I run tune2fs -l:

 tune2fs 1.22, 22-Jun-2001 for EXT2 FS 0.5b, 95/08/09
 Filesystem volume name:   <none>
 Last mounted on:          <not available>
 Filesystem UUID:          f5719854-d253-4809-9504-84850c44a796
 Filesystem magic number:  0xEF53
 Filesystem revision #:    1 (dynamic)
 Filesystem features:      has_journal filetype needs_recovery sparse_super
 Filesystem state:         clean

This is my root FS, and it is mounted, should this be clean?


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