Re: How to know whether disks "handle flush requests correctly"

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Josef Bacik's message of 2011-05-06 09:10:23 -0400:
> On 05/06/2011 05:13 AM, Paul Schroeder wrote:
> > The btrfs wiki Main Page warns that "it is currently possible to corrupt
> > a filesystem irrecoverably if your machine crashes or loses power on disks
> > that don't handle flush requests correctly."
> >
> > How do you know if this applies to your drives? Is there a way to test it,
> > or a model list, or are newer SATA drives (magnetic, not SSDs) always ok?
> > Does it depend on the controller? (I have a SiI 3114, latest BIOS.)
> >
> > I would also be using btrfs on top of dm-crypt (with the latest release
> > kernel). Some kernel versions ago, the message that write barriers aren't
> > supported disappeared; can I assume the device mapper / dm-crypt is not a
> > problem with regards to flushing?
> >
> 
> Yeah if you don't see those messages you can be fairly certain you are 
> ok.  Thanks,

The easiest way to tell for sure is to do two tests:

dd if=/dev/zero of=/mnt/foo bs=4K count=10000
dd if=/dev/zero of=/mnt/foo bs=4K count=10000 oflag=sync

Run this with the filesystem mounted normally and again with the
filesystem mounted -o nobarrier.  -o nobarrier should be dramatically
and hugely faster, almost like we're not writing to the disk at all.

-chris