Btrfs devel - May 2011 - How to know whether disks "handle flush requests correctly"

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?

Paul
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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,

Josef
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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
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