Re: Is metadata redundant over more than one drive with raid0 too?

[Marc MERLIN <marc@merlins.org>]

On Sun, May 04, 2014 at 08:57:19AM +0200, Brendan Hide wrote:
> Hi, Marc
> 
> Raid0 is not redundant in any way. See inline below.
 
Thanks for clearing things up.

> >But now I have 2 questions
> >1) btrfs has two copies of all metadata on even a single drive, correct?
> 
> Only when *specifically* using -m dup (which is the default on a
> single non-SSD device), will there be two copies of the metadata
> stored on a single device. This is not recommended when using

Ah, so -m dup is default like I thought, but not on SSD?
Ooops, that means that my laptop does not have redundant metadata on its
SSD like I thought. Thanks for the heads up.
Ah, I see the man page now "This is because SSDs can remap blocks
internally so duplicate blocks could end up in the same erase block
which negates the benefits of doing metadata duplication."

> multiple devices as it means one device failure will likely cause
> critical loss of metadata. 

That's the part where I'm not clear:

What's the difference between -m dup and -m raid1
Don't they both say 2 copies of the metadata?
Is -m dup only valid for a single drive, while -m raid1 for 2+ drives?

> >If so, and I have a -d raid0 -m raid0 filesystem, are both copies of the
> >metadata on the same drive or is btrfs smart enough to spread out
> >metadata copies so that they're not on the same drive?
> 
> This will mean there is only a single copy, albeit striped across
> the drives.

Ok, so -m raid0 only means a single copy of metadata, thanks for
explaining.

> good for redundancy. A total failure of a single device will mean
> any large files will be lost and only files smaller than the default
> per-disk stripe width (I believe this used to be 4K and is now 16K -
> I could be wrong) stored only on the remaining disk will be
> available.
 
Gotcha, thanks for confirming, so -m raid1 -d raid0 really only protects
against metadata corruption or a single block loss, but otherwise if you
lost a drive in a 2 drive raid0, you'll have lost more than just half
your files.

> The scenario you mentioned at the beginning, "if I lose a drive,
> I'll still have full metadata for the entire filesystem and only
> missing files" is more applicable to using "-m raid1 -d single".
> Single is not geared towards performance and, though it doesn't
> guarantee a file is only on a single disk, the allocation does mean
> that the majority of all files smaller than a chunk will be stored
> on only one disk or the other - not both.

Ok, so in other words:
-d raid0: if you one 1 drive out of 2, you may end up with small files
and the rest will be lost

-d single: you're more likely to have files be on one drive or the
other, although there is no guarantee there either.

Correct?

Thanks,
Marc
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