[patch] Re: writing file to disk: not as easy as it looks

[Pavel Machek]

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@xxxxxxx>

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
 ==========





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