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Gmane
From: Pavel Machek <pavel <at> ucw.cz>
Subject: ext2/3: document conditions when reliable operation is possible
Newsgroups: gmane.comp.file-systems.ext4
Date: Thursday 12th March 2009 09:21:14 UTC (over 8 years ago)
Not all block devices are suitable for all filesystems. In fact, some
block devices are so broken that reliable operation is pretty much
impossible. Document stuff ext2/ext3 needs for reliable operation.

Signed-off-by: Pavel Machek 

diff --git a/Documentation/filesystems/expectations.txt
b/Documentation/filesystems/expectations.txt
new file mode 100644
index 0000000..9c3d729
--- /dev/null
+++ b/Documentation/filesystems/expectations.txt
@@ -0,0 +1,47 @@
+Linux block-backed filesystems can only work correctly when several
+conditions are met in the block layer and below (disks, flash
+cards). Some of them are obvious ("data on media should not change
+randomly"), some are less so.
+
+Write errors not allowed (NO-WRITE-ERRORS)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Writes to media never fail. Even if disk returns error condition
+during write, filesystems can't handle that correctly, because success
+on fsync was already returned when data hit the journal.
+
+	Fortunately writes failing are very uncommon on traditional 
+	spinning disks, as they have spare sectors they use when write
+	fails.
+
+Sector writes are atomic (ATOMIC-SECTORS)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+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 all linux filesystems 
+	I know. 
+
+		An inherent problem with using flash as a normal block
+		device is that the flash erase size is bigger than
+		most filesystem sector sizes.  So when you request a
+		write, it may erase and rewrite the next 64k, 128k, or
+		even a couple megabytes on the really _big_ ones.
+
+		If you lose power in the middle of that, filesystem
+		won't notice that data in the "sectors" _around_ the
+		one your were trying to write to got trashed.
+
+	Because RAM tends to fail faster than rest of system during 
+	powerfail, special hw killing DMA transfers may be neccessary;
+	otherwise, disks may write garbage during powerfail.
+	Not sure how common that problem is on generic PC machines.
+
+	Note that atomic write is very hard to guarantee for RAID-4/5/6,
+	because it needs to write both changed data, and parity, to 
+	different disks.
+
+
+
diff --git a/Documentation/filesystems/ext2.txt
b/Documentation/filesystems/ext2.txt
index 4333e83..b09aa4c 100644
--- a/Documentation/filesystems/ext2.txt
+++ b/Documentation/filesystems/ext2.txt
@@ -338,27 +339,25 @@ 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.
 
+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:
+
+* write errors not allowed
+
+* sector writes are atomic
+
+(see expectations.txt; note that most/all linux block-based
+filesystems have similar expectations)
+
+* write caching is disabled. ext2 does not know how to issue barriers
+  as of 2.6.28. hdparm -W0 disables it on SATA disks.
+
 Journaling
-----------
-
-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.
 
diff --git a/Documentation/filesystems/ext3.txt
b/Documentation/filesystems/ext3.txt
index 9dd2a3b..02a9bd5 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -188,6 +200,27 @@ 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:
+
+* write errors not allowed
+
+* sector writes are atomic
+
+(see expectations.txt; note that most/all linux block-based
+filesystems have similar expectations)
+
+* 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). 
+
+	   hdparm -I reports disk features. If you have "Native
+	   Command Queueing" is the feature you are looking for.
 
 References
 ==========

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