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Re: Experiences with BTRFS -- is it mature enough for enterprise use?




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On Mon, Jan 01, 2018 at 06:01:20PM -0800, David Christensen wrote:
> On 12/31/17 14:45, Sven Hartge wrote:
> >David Christensen <dpchrist@xxxxxxxxxxxxxxxx> wrote:
> >>     $ man 4 md
> >
> >>     SCRUBBING AND MISMATCHES
> >>     ...
> >>        If check was used, then no action is taken to handle the mismatch,  it
> >>        is  simply  recorded.   If  repair  was  used,  then a mismatch will be
> >>        repaired in the same way that resync repairs arrays.   For RAID5/RAID6
> >>        new parity blocks are written.  For RAID1/RAID10, all but one block are
> >>        overwritten with the content of that one block.
> >
> >
> >>I wonder how md picks "that one block"?
> >
> >Only if one drives reports an error. Then data from the good block is
> >used to overwrite the bad block, hoping the drive remaps the sector and
> >everything is fine again.
> >
> >If both devices report no error but differing data has been read,
> >MD-RAID1 can't know which block is good.
> >
> >MD-RAID5/6 could calculate all parity combinations and use the data a
> >majority agrees upon. (I don't know if it does it, though).
> >
> >I tried looking at the Kernel RAID code, but I must admit: it is all
> >Esperanto to me, the code is far too low level for me to understand.
> 
> That's why "programming systems product" [1] includes architectural,
> functional, design, construction, etc., documentation.
> 
> 
> FreeBSD is better is this regard [2].

Look for documentation in the right shelf. Hint: it's called
Documentation (there are books in there, not nuts and bolts).

As an example: I'm on 4.9.0 (plus some assorted Debian-specific
patches). If I don't want to download the whole kaboodle (although
that would be a good idea), I might be tempted to use the nice
gitweb interface at git.kernel.org:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/?h=v4.9

The "tree" view (provided above for your convenience, but you
mith be able to click your way through), you might perhaps find
docs in or around Documentation/md.txt.

Reading source only helps if you have a rough idea on what's going
on.

Cheers
- -- t
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