Re: Renaming MD devices (metadata=1.1)

[NeilBrown <neilb@suse.de>]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Wed, 11 Jan 2012 11:01:33 -0500 Gilbert Kowarzyk <kowarzyk@grm.polymtl.ca>
wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Hello,
> 
> >> Why do we have these choices, and when should each be used?
> > 
> > 1.0 (at the end) is best for RAID1 when booting from the device as
> > the boot-load just doesn't see the RAID at all.
> > 
> > 1.1 (at the start) is generally good because various different
> > things have metadata at the start, so there is no chance of
> > confusion about what is using that devices - each possible users
> > will overwrite the metadata of the others. It is also easier to
> > make the devices larger if you don't need to move the metadata.
> > 
> > 1.2 (4K from the start) has most of the benefits for 1.1 but works
> > for booting with newer boot loaders - they still want sector 0 for
> > a boot sector but understand the md superblock.
> 
> I see. Makes also loads of sense.
> 
> I had two more quick questions:
> 
> 1.- when would one want to turn off bitmaps? I turned them on, but I
> haven't found why one may want them off (or for what they should be
> turned off).

There can be a performance impact - writes can be a bit slower with bitmaps.
If that is more important to you that resync speed after a crash you might
turn them off.
To change the chunk-size of the bitmap you need to turn them off, then on
again.
If you want to reshape the array you currently need to turn the bitmap off
first (because the bitmap doesn't automatically resize).
You turn the bitmap off with
   mdadm --grow /dev/mdXX --bitmap=none

> 
> 2.- if I ever have a mismatch_cnt different than zero, how would I go
> about finding which drive has the correct info, which was the one that
> got corrupted, and how to obtain the correct one?

"correct" is not a well defined concept here.

> In the past I've changed the "CHECK" to "REPAIR" in the configuration
> script that checks the RAID arrays periodically
> (/etc/sysconfig/raid-check), but from what I understood it's a bit of
> luck which one the automatic script will choose out of the two drives
> (assuming raid1). Should I let it be automatic (with "repair"), and if
> not (if I have to check which one is correct manually), where could I
> find information to read about how to do this (I am assuming the
> answer may be long)?

This:  http://neil.brown.name/blog/20100211050355
touches on the topic ... and is long.


NeilBrown
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.18 (GNU/Linux)
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=cpNq
-----END PGP SIGNATURE-----