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

From: david
Date: Thu Aug 27 2009 - 02:55:18 EST

On Thu, 27 Aug 2009, Rob Landley wrote:

On Wednesday 26 August 2009 07:28:13 Theodore Tso wrote:
On Wed, Aug 26, 2009 at 01:17:52PM +0200, Pavel Machek wrote:
Metadata takes up such a small part of the disk that fscking
it and finding it to be OK is absolutely no guarantee that
the data on the filesystem has not been horribly mangled.

Personally, what I care about is my data.

The metadata is just a way to get to my data, while the data
is actually important.

Personally, I care about metadata consistency, and ext3 documentation
suggests that journal protects its integrity. Except that it does not
on broken storage devices, and you still need to run fsck there.

Caring about metadata consistency and not data is just weird, I'm
sorry. I can't imagine anyone who actually *cares* about what they
have stored, whether it's digital photographs of child taking a first
step, or their thesis research, caring about more about the metadata
than the data. Giving advice that pretends that most users have that
priority is Just Wrong.

I thought the reason for that was that if your metadata is horked, further
writes to the disk can trash unrelated existing data because it's lost track
of what's allocated and what isn't. So back when the assumption was "what's
written stays written", then keeping the metadata sane was still darn
important to prevent normal operation from overwriting unrelated existing

Then Pavel notified us of a situation where interrupted writes to the disk can
trash unrelated existing data _anyway_, because the flash block size on the 16
gig flash key I bought retail at Fry's is 2 megabytes, and the filesystem thinks
it's 4k or smaller. It seems like what _broke_ was the assumption that the
filesystem block size >= the disk block size, and nobody noticed for a while.
(Except the people making jffs2 and friends, anyway.)

Today we have cheap plentiful USB keys that act like hard drives, except that
their write block size isn't remotely the same as hard drives', but they
pretend it is, and then the block wear levelling algorithms fuzz things
further. (Gee, a drive controller lying about drive geometry, the scsi crowd
should feel right at home.)

actually, you don't know if your USB key works that way or not. Pavel has ssome that do, that doesn't mean that all flash drives do

when you do a write to a flash drive you have to do the following items

1. allocate an empty eraseblock to put the data on

2. read the old eraseblock

3. merge the incoming write to the eraseblock

4. write the updated data to the flash

5. update the flash trnslation layer to point reads at the new location instead of the old location.

now if the flash drive does things in this order you will not loose any previously written data.

if the flash drive does step 5 before it does step 4, then you have a window where a crash can loose data (and no btrfs won't survive any better to have a large chunk of data just disappear)

it's possible that some super-cheap flash drives skip having a flash translation layer entirely, on those the process would be

1. read the old data into ram

2. merge the new write into the data in ram

3. erase the old data

4. write the new data

this obviously has a significant data loss window.

but if the device doesn't have a flash translation layer, then repeated writes to any one sector will kill the drive fairly quickly. (updates to the FAT would kill the sectors the FAT, journal, root directory, or superblock lives in due to the fact that every change to the disk requires an update to this file for example)

Now Pavel's coming back with a second situation where RAID stripes (under
certain circumstances) seem to have similar granularity issues, again breaking
what seems to be the same assumption. Big media use big chunks for data, and
media is getting bigger. It doesn't seem like this problem is going to
diminish in future.

I agree that it seems like a good idea to have BIG RED WARNING SIGNS about
those kind of media and how _any_ journaling filesystem doesn't really help
here. So specifically documenting "These kinds of media lose unrelated random
data if writes to them are interrupted, journaling filesystems can't help with
this and may actually hide the problem, and even an fsck will only find
corrupted metadata not lost file contents" seems kind of useful.

I think an update to the documentation is a good thing (especially after learning that a raid 6 array that has lost a single disk can still be corrupted during a powerfail situation), but I also agree that Pavel's wording is not detailed enough

That said, ext3's assumption that filesystem block size always >= disk update
block size _is_ a fundamental part of this problem, and one that isn't shared
by things like jffs2, and which things like btrfs might be able to address if
they try, by adding awareness of the real media update granularity to their
node layout algorithms. (Heck, ext2 has a stripe size parameter already.
Does setting that appropriately for your raid make this suck less? I haven't
heard anybody comment on that one yet...)

I thought that that assumption was in the VFS layer, not in any particular filesystem

David Lang
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