lkml.org 
[lkml]   [2008]   [Dec]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[patch] Re: writing file to disk: not as easy as it looks
Hi!

> > > Heck, if you have a hiccup while writing an inode table block out to
> > > disk (for example a power failure at just the wrong time), so the
> > > memory (which is more voltage sensitive than hard drives) DMA's
> > > garbage which gets written to the inode table, you could lose a large
> > > number of adjacent inodes when garbage gets splatted over the inode
> > > table.
> >
> > Ok, "memory failed before disk" is ... bad hardware.
>
> It's PC class hardware. Live with it. Back when SGI made their own
> hardware, they noticed this problem, and so they wired up their SGI
> machines with powerfail interrupts, and extra big capacitors in
> their

Seems like bad hardware is very common indeed. Anyway, I guess it
would be fair to document what ext3 expects from disk subsystem for
safe operation. Does that summary sound correct/fair?

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

diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
index 9dd2a3b..3855fbd 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -188,6 +188,34 @@ mke2fs: create a ext3 partition with th
debugfs: ext2 and ext3 file system debugger.
ext2online: online (mounted) ext2 and ext3 filesystem resizer

+Requirements
+============
+
+Ext3 expects disk/storage subsystem to behave sanely. On sanely
+behaving disk subsystem, data that have been successfully synced will
+stay on the disk. Sane means:
+
+* writes to media never fail. Even if disk returns error condition during
+ write, ext3 can't handle that correctly, because success on fsync was already
+ returned when data hit the journal.
+
+ (Fortunately writes failing are very uncommon on disks, as they
+ have spare sectors they use when write fails.)
+
+* either whole sector is correctly written or nothing is written during
+ powerfail.
+
+ (Unfortuantely, all the cheap USB/SD flash cards I seen do behave
+ like this, and are unsuitable for ext3. Because RAM tends to fail
+ faster than rest of system during powerfail, special hw killing
+ DMA transfers may be neccessary. Not sure how common that problem
+ is on generic PC machines).
+
+* either write caching is disabled, or hw can do barriers and they are enabled.
+
+ (Note that barriers are disabled by default, use "barrier=1"
+ mount option after making sure hw can support them).
+

References
==========




--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


\
 
 \ /
  Last update: 2008-12-15 11:25    [W:0.119 / U:0.348 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site