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

From: Ric Wheeler
Date: Tue Aug 25 2009 - 09:38:44 EST

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.


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?

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...


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