Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: Ric Wheeler
Date: Tue Aug 25 2009 - 21:16:10 EST

On 08/25/2009 09:00 PM, Theodore Tso wrote:
On Tue, Aug 25, 2009 at 08:31:21PM -0400, Ric Wheeler wrote:
You are simply incorrect, Ted did not say that ext3 does not work
with MD raid5.
I will let Ted clarify his text on his own, but the quoted text says "...
have potential...".

Why not ask Neil if he designed MD to not work properly with ext3?
So let me clarify by saying the following things.

1) Filesystems are designed to expect that storage devices have
certain properties. These include returning the same data that you
wrote, and that an error when writing a sector, or a power failure
when writing sector, should not be amplified to cause collateral
damage with previously succfessfully written sectors.

2) Degraded RAID 5/6 filesystems do not meet these properties.
Neither to cheap flash drives. This increases the chances you can
lose, bigtime.

I agree with the whole write up outside of the above - degraded RAID does meet this requirement unless you have a second (or third, counting the split write) failure during the rebuild.

Note that the window of exposure during a RAID rebuild is linear with the size of your disk and how much you detune the rebuild...


3) Does that mean that you shouldn't use ext3 on RAID drives? Of
course not! First of all, Ext3 still saves you against kernel panics
and hangs caused by device driver bugs or other kernel hangs. You
will lose less data, and avoid needing to run a long and painful fsck
after a forced reboot, compared to if you used ext2. You are making
an assumption that the only time running the journal takes place is
after a power failure. But if the system hangs, and you need to hit
the Big Red Switch, or if you using the system in a Linux High
Availability setup and the ethernet card fails, so the STONITH ("shoot
the other node in the head") system forces a hard reset of the system,
or you get a kernel panic which forces a reboot, in all of these cases
ext3 will save you from a long fsck, and it will do so safely.

Secondly, what's the probability of a failure causes the RAID array to
become degraded, followed by a power failure, versus a power failure
while the RAID array is not running in degraded mode? Hopefully you
are running with the RAID array in full, proper running order a much
larger percentage of the time than running with the RAID array in
degraded mode. If not, the bug is with the system administrator!

If you are someone who tends to run for long periods of time in
degraded mode --- then better get a UPS. And certainly if you want to
avoid the chances of failure, periodically scrubbing the disks so you
detect hard drive failures early, instead of waiting until a disk
fails before letting the rebuild find the dreaded "second failure"
which causes data loss, is a d*mned good idea.

Maybe a random OS engineer doesn't know these things --- but trust me
when I say a competent system administrator had better be familiar
with these concepts. And someone who wants their data to be reliably
stored needs to do some basic storage engineering if they want to have
long-term data reliability. (That, or maybe they should outsource
their long-term reliable storage some service such as Amazon S3 ---
see Jeremy Zawodny's analysis about how it can be cheaper, here:

But we *do* need to be careful that we don't write documentation which
is ends up giving users the wrong impression. The bottom line is that
you're better off using ext3 over ext2, even on a RAID array, for the
reasons listed above.

Are you better off using ext3 over ext2 on a crappy flash drive?
Maybe --- if you are also using crappy proprietary video drivers, such
as Ubuntu ships, where every single time you exit a 3d game the system
crashes (and Ubuntu users accept this as normal?!?), then ext3 might
be a better choice since you'll reduce the chance of data loss when
the system locks up or crashes thanks to the aforemention crappy
proprietary video drivers from Nvidia. On the other hand, crappy
flash drives *do* have really bad write amplification effects, where a
4K write can cause 128k or more worth of flash to be rewritten, such
that using ext3 could seriously degrade the lifetime of said crappy
flash drive; furthermore, the crappy flash drives have such terribly
write performance that using ext3 can be a performance nightmare.
This of course, doesn't apply to well-implemented SSD's, such as the
Intel's X25-M and X18-M. So here your mileage may vary. Still, if
you are using crappy proprietary drivers which cause system hangs and
crashes at a far greater rate than power fail-induced unclean
shutdowns, ext3 *still* might be the better choice, even with crappy
flash drives.

The best thing to do, of course, is to improve your storage stack; use
competently implemented SSD's instead of crap flash cards. If your
hardware RAID card supports a battery option, *get* the battery. Add
a UPS to your system. Provision your RAID array with hot spares, and
regularly scrub (read-test) your array so that failed drives can be
detected early. Make sure you configure your MD setup so that you get
e-mail when a hard drive fails and the array starts running in
degraded mode, so you can replace the failed drive ASAP.

At the end of the day, filesystems are not magic. They can't
compensate for crap hardware, or incompetently administered machines.

- Ted
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