RE: Question on the fs choice

Sorry for following up my own post, but this is a very interesting talk on EXT3 which has application to all journaling file systems...
  http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html

A very interesting quote:
  Now, disks these days actually make these guarantees. If you start a write
  operation to a disk, then even if the power fails in the middle of that
  sector write, the disk has enough power available, and it can actually steal
  power from the rotational energy of the spindle; it has enough power to
  complete the write of the sector that's being written right now. In all
  cases, the disks make that guarantee. [23m, 41s]

This is Wolfgang's and my point about CF: the CF itself does not have sufficient energy storage inside it to guarantee completion of a write operation.  In addition, our local experience with flash chips is that a power fail during a write operation can scribble on RANDOM locations in the flash chip.  The intuitive assumption is that a power fail during a CF write will corrupt ONLY the sector that is the target of the write operation.  This is NOT a good assumption.

gvb

-----Original Message-----
From: owner-linuxppc-embedded@lists.linuxppc.org
[mailto:owner-linuxppc-embedded@lists.linuxppc.org]On Behalf Of
VanBaren, Gerald (AGRE)
Sent: Wednesday, May 12, 2004 8:37 AM
To: linuxppc-embedded@lists.linuxppc.org
Subject: RE: Question on the fs choice



Journaling file systems don't cure corruption on power fail,
they just make the power fail recovery faster by eliminating
the full fsck disk scan and significantly reduce the
probability of corruption due to the transactional nature.
The assumption with a journaling file system is that the
power fail only corrupted the last (partial) transaction.
With normal usage patterns this is a good assumption
(spinning disk physics probably help, but write caches in
disks are going to hurt).  With CF under heavy power failure
cycles, this might not be as good an assumption.

There was a slashdot item yesterday pointing to a benchmark
of various journaling file systems.  In the discussion, one
point that was made was that ext3 journals the data as well
as the metadata by default where the other journaling file
systems journal only the metadata (directory info).  I
presume this was accurate (hey, it was on ./ which guarantees
accuracy, right? ;-).  Whichever fs you pick, you probably
want to journal the data as well as the metadata.

A suggestion is to have multiple partitions on your CF and
use RAID-3 or RAID-5.  The assumption here is that a power
fail-induced physical corruption would corrupt only one of
the partitions which could then be rebuilt due to the RAID
redundancy.  The journaled file system would then eliminate
the fsck on start up and make the file system itself
consistant and the RAID redundancy would (presumably) fix any
low level errors that the unexpected power fail could induce.

...or maybe a journaling file system is good enough.

gvb

quoted

-----Original Message-----
From: owner-linuxppc-embedded@lists.linuxppc.org
[mailto:owner-linuxppc-embedded@lists.linuxppc.org]On

Behalf Of Zajac

quoted

Adam-AAZ004
Sent: Tuesday, May 11, 2004 7:20 PM
To: 'linuxppc-embedded@lists.linuxppc.org'
Subject: Question on the fs choice



Hello,

We've built a custom board based on MPC8540 that runs Linux
OS and utilizes
compact flash card to store the kernel on a raw-data
partition (read-only)
and root file system on an Ext2 partition (read-write). As
Ext2 is not a
journaling FS, in case of any power failure the file system
is not cleanly
unmounted and the fscheck complains upon a system start-up.
Also, sometimes the file system partition gets corrupted when
we power-cycle
the card instead of shutting the system down gracefully (the
card is build
to be "hot-swappable" so we test it on purpose).

I saw Wolfgang Denx's post discouraging any use of the CF

cards for an

quoted

embedded platform where write operations are performed under
power-failure
prone environment. Unfortunately, this is the reality we're
facing on our
card.

I'm seeking some help with selecting the file system that
would survive
power failure if the CF card didn't get damaged during a
write cycle (we're
gonna perform a thorough testing to assess the severity of
that issue).

I've seen some articles suggesting one of the journaling
systems, like Ext3,
XFS or ReiserFS.

Has anybody successfully implemented any of these FS systems
on an embedded
platform equipped with a compact flash card working as a

main storage

quoted

medium?

Any help would be greatly appreciated.

Adam Zajac
Motorola Inc, Global Telecom Solutions Sector
5555 N Beach St, Mailstop 8E, Fort Worth, TX 76137-2794
Phone: (817) 245-7963

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