[CentOS] Some 3Ware questions

[Bryan J. Smith]

Remco Barendse <redhat at barendse.to> wrote:
> That's not entirely correct. The 3Ware disks almost use
> standard raid format,

There is no "standard RAID format."

Different vendors -- be it hardware or FRAID -- as well OS
software organizations use different offsets, blocking,
etc... for RAID-0, 10, 3, 4, 5, etc...  But the overwhelming
majority of RAID implementations have a "meta-data block" or
set of blocks that describe this organization.  It's done so
drivers, firmware or even other cards from the same
vendor/code can read the disk organization.

The Device Mapper (DM) work is able to read some of these
meta-data blocks to understand RAID-0, 10 and even "1e" (2
disc RAID-10, long story).

About the _only_ "industry standard" is RAID-1.  I haven't
seen a vendor yet who didn't do straight mirroring -- 1
sector to 1 sector.

> it's just the raid block on each disk that is written at
> the end of the disk I believe instead of at the beginning.

Again, you are talking about the meta-data block(s).
That's different.  3Ware adds those, yes.
It's mainly for managing the disk.
Most vendors do put them at the end, yes.
But vendors differ on their meta-data blocks.
But most have been reverse engineered and documented.

I'm talking about being able to take a standalone disk and
put it into a RAID-1 array on a 3Ware card.  You can because
RAID-1, unlike RAID-0, 10 or 5 on a 3Ware card, does _not_
stripe data in 32KiB (default) blocks.

> About a year a go we rescued all data from a series of
> PATA disks from a friend of mine when his 3Ware went bust.
> We were able to start the array in software raid with 
> the right parameters. Google was my friend then

Was it RAID-1?  Or RAID-10 or 5?

RAID-1 volumes are _extremely_easy_ to read.

RAID-0, 10 or (not yet?) 5 requires Device Mapper (DM) or
another software implementation _must_ read the 3Ware
meta-data blocks so it can understand the striping/blocking
organization.  Not being able to read the meta-data blocks
will prevent you from finding the _exact_ striping positions
and blocking size (32KiB is the default) for RAID-0, 10 and
5.

But RAID-1 is just straight mirroring.  It's not as important
if you can't read the meta-data block.


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