Re: [patch] ext2/3: document conditions when reliable operation ispossible

[Ric Wheeler]

On 08/25/2009 05:42 AM, Pavel Machek wrote:
> On Mon 2009-08-24 20:08:42, Theodore Tso wrote:
> > On Tue, Aug 25, 2009 at 01:00:36AM +0200, Pavel Machek wrote:
> > > Then to answer your question... ext2. You expect to run fsck after
> > > unclean shutdown, and you expect to have to solve some problems with
> > > it. So the way ext2 deals with the flash media actually matches what
> > > the user expects. (*)
> >
> > But if the 256k hole is in data blocks, fsck won't find a problem,
> > even with ext2.
>
> True.
>
> > And if the 256k hole is the inode table, you will *still* suffer
> > massive data loss.  Fsck will tell you how badly screwed you are, but
> > it doesn't "fix" the disk; most users don't consider questions of the
> > form "directory entry<precious-thesis-data>  points to trashed inode,
> > may I delete directory entry?" as being terribly helpful.  :-/
>
> Well it will fix the disk in the end. And no, "directory entry
> <precious-thesis-data>  points to trashed inode, may I delete directory
> entry?" is not _terribly_ helpful, but it is slightly helpful and
> people actually expect that from ext2.
>
> > Maybe this came as a surprise to you, but anyone who has used a
> > compact flash in a digital camera knows that you ***have*** to wait
> > until the led has gone out before trying to eject the flash card.  I
> > remember seeing all sorts of horror stories from professional
> > photographers about how they lost an important wedding's day worth of
> > pictures with the attendant commercial loss, on various digital
> > photography forums.  It tends to be the sort of mistake that digital
> > photographers only make once.
>
> It actually comes as surprise to me. Actually yes and no. I know that
> digital cameras use VFAT, so pulling CF card out of it may do bad
> thing, unless I run fsck.vfat afterwards. If digital camera was using
> ext3, I'd expect it to be safely pullable at any time.
>
> Will IBM microdrive do any difference there?
>
> Anyway, it was not known to me. Rather than claiming "everyone knows"
> (when clearly very few people really understand all the details), can
> we simply document that?
> 									Pavel

I really think that the expectation that all OS's (windows, mac, even your ipod) 
all teach you not to hot unplug a device with any file system. Users have an 
"eject" or "safe unload" in windows, your iPod tells you not to power off or 
disconnect, etc.

I don't object to making that general statement - "Don't hot unplug a device 
with an active file system or actively used raw device" - but would object to 
the overly general statement about ext3 not working on flash, RAID5 not working, 
etc...

ric



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