document ext3 requirements

[Pavel Machek <pavel@suse.cz>]

Using ext3 is only safe if storage subsystem meets certain
criteria. Document those.

Errors=remount-ro is documented as default, but superblock setting
overrides that and mkfs defaults to errors=continue... so the default
is errors=continue in practice.

readonly mount does actually write to the media in some cases. Document that.

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

diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
index 9dd2a3b..74a73b0 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -14,6 +14,9 @@ Options
 When mounting an ext3 filesystem, the following option are accepted:
 (*) == default
 
+ro			Note that ext3 will replay the journal (and thus write
+			to the partition) even when mounted "read only".
+
 journal=update		Update the ext3 file system's journal to the current
 			format.
 
@@ -95,6 +98,8 @@ debug			Extra debugging information is sent to syslog.
 errors=remount-ro(*)	Remount the filesystem read-only on an error.
 errors=continue		Keep going on a filesystem error.
 errors=panic		Panic and halt the machine if an error occurs.
+			(Note that default is overriden by superblock
+			setting on most systems).
 
 data_err=ignore(*)	Just print an error message if an error occurs
 			in a file data buffer in ordered mode.
@@ -188,6 +193,34 @@ mke2fs: 	create a ext3 partition with the -j flag.
 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, none of 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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