Date: Tue, 15 Jan 2008 21:16:53 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Theodore Tso <tytso@mit.edu>, Al Boldi <a1426z@gawab.com>,
       Valerie Henson <val.henson@gmail.com>, Rik van Riel <riel@redhat.com>,
       linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Message-ID: <20080115201653.GA5639@elf.ucw.cz>
References: <200801090022.55589.a1426z@gawab.com> <200801090740.12989.a1426z@gawab.com> <70b6f0bf0801082345vf57951ey642e35c3d6e5194f@mail.gmail.com> <200801091452.14890.a1426z@gawab.com> <20080112145140.GB6751@mit.edu> <20080113171916.GB4132@ucw.cz> <20080113174125.5f39ac64@lxorguk.ukuu.org.uk>
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Hi!

> > What are ext3 expectations of disk (is there doc somewhere)? For
> > example... if disk does not lie, but powerfail during write damages
> > the sector -- is ext3 still going to work properly?
> 
> Nope. However the few disks that did this rapidly got firmware updates
> because there are other OS's that can't cope.
> 
> > If disk does not lie, but powerfail during write may cause random
> > numbers to be returned on read -- can fsck handle that?
> 
> most of the time. and fsck knows about writing sectors to remove read
> errors in metadata blocks.
> 
> > What abou disk that kills 5 sectors around sector being written during
> > powerfail; can ext3 survive that?
> 
> generally. Note btw that for added fun there is nothing that guarantees
> the blocks around a block on the media are sequentially numbered. The
> usually are but you never know.

Ok, should something like this be added to the documentation?

It would be cool to be able to include few examples (modern SATA disks
support bariers so are safe, any IDE from 1989 is unsafe), but I do
not know enough about hw...

Signed-off-by: Pavel Machek <pavel@suse.cz>

diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
index b45f3c1..adfcc9d 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -183,6 +183,18 @@ mke2fs: 	create a ext3 partition with th
 debugfs: 	ext2 and ext3 file system debugger.
 ext2online:	online (mounted) ext2 and ext3 filesystem resizer
 
+Requirements
+============
+
+Ext3 needs disk that does not do write-back caching or disk that
+supports barriers and Linux configuration that can use them.
+
+* if disk damages the sector being written during powerfail, ext3
+  can't cope with that.  Fortunately, such disks got firmware updates
+  to fix this long time ago.
+
+* if disk writes random data during powerfail, ext3 should survive
+  that most of the time.
 
 References
 ==========


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