[patch] document that ext2 can't handle barriers

From: Pavel Machek
Date: Tue Aug 25 2009 - 18:27:40 EST

Document things ext2 expects from storage filesystems, and the fact
that it can not handle barriers. Also remove jounaling description, as
that's really ext3 material.

Signed-off-by: Pavel Machek <pavel@xxxxxx>

diff --git a/Documentation/filesystems/ext2.txt b/Documentation/filesystems/ext2.txt
index 67639f9..e300ca8 100644
--- a/Documentation/filesystems/ext2.txt
+++ b/Documentation/filesystems/ext2.txt
@@ -338,27 +339,17 @@ enough 4-character names to make up unique directory entries, so they
have to be 8 character filenames, even then we are fairly close to
running out of unique filenames.

+Ext2 expects disk/storage subsystem not to return write errors.
+It also needs write caching to be disabled for reliable fsync
+operation; ext2 does not know how to issue barriers as of
+2.6.31. hdparm -W0 disables it on SATA disks.
-A journaling extension to the ext2 code has been developed by Stephen
-Tweedie. It avoids the risks of metadata corruption and the need to
-wait for e2fsck to complete after a crash, without requiring a change
-to the on-disk ext2 layout. In a nutshell, the journal is a regular
-file which stores whole metadata (and optionally data) blocks that have
-been modified, prior to writing them into the filesystem. This means
-it is possible to add a journal to an existing ext2 filesystem without
-the need for data conversion.
-When changes to the filesystem (e.g. a file is renamed) they are stored in
-a transaction in the journal and can either be complete or incomplete at
-the time of a crash. If a transaction is complete at the time of a crash
-(or in the normal case where the system does not crash), then any blocks
-in that transaction are guaranteed to represent a valid filesystem state,
-and are copied into the filesystem. If a transaction is incomplete at
-the time of the crash, then there is no guarantee of consistency for
-the blocks in that transaction so they are discarded (which means any
-filesystem changes they represent are also lost).
Check Documentation/filesystems/ext3.txt if you want to read more about
ext3 and journaling.

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