>> Like any RAID setup, you need "N+1" drives
>
>only for RAID5, RAID4, and RAID3

Agreed.  I overstated/overused the term "RAID" here.

>You describe RAID4, probably the least commonly used RAID, since it
>means only one write request can be satisfied at once, as there is a
>only single drive doing parity writes.  Much better is to rotate the
>parity from one drive to the next with each write (RAID5), meaning that
>write requests can be satisfied concurrently.

Ummm, we're talking about tapes here.  Multiple concurrent writes doesn't
mean a whole lot :-).

I'm not saying rotating the parity wouldn't be a good idea.  Be my
guest to add that and send in a patch :-).

But note that doing so is going to make a "bare metal" recovery that
much harder if amdd isn't around to help.

>Besides which, there is no such rule on disk arrays, even with RAID4
>(that `N' must be a power of two).  ...

I didn't say it needed to be a power of two.  I said:

  ... you need "N+1" drives where "N" is a divisor of the
  data read/write size.

I then said:

  ... the tape code is getting (by default)
  32KByte chunks of data, so "N" must be a power of 2.

For the *specific* case of RAIT with Amanda and the "normal" block size,
power of 2 is needed.  For anything else (including using amdd for other
purposes), anything that is an even divisor is required.

>> ...  I'm almost positive you can use 2 drives to achieve mirroring
>> (one data, one parity, which with a single drive turns out to be a
>> copy of the original).
>
>Using mirroring doesn't employ any parity calculations (why use CPU when
>you don't have to?).  It merely writes all data identically to two
>drives.

Again, that's not what I said.  I said using 2 drives with RAIT *achieves*
mirroring, and I meant it was a convenient side effect of how things
worked out.  Some folks have asked for mirroring.  This can accomplish
that.

Also, the exclusive or is not performed in the special end case of one
data drive, so no (well, minimal) CPU is used to get mirroring via RAIT.

>It's unfortunate that he doesn't read this list :)

And how would we know that?

I've been "off the list" for several months because I've been too busy.
It doesn't have any relationship to how I'm using Amanda or whether I've
been getting the contents of the list.

>And since no one else replied I will have to conclude that he is the
>only one using RAIT in production.  ...

Ummm, I don't follow that logic at all.  There are over 1000 addresses
subscribed to amanda-users.  This is a "chatty" place, but not *that*
chatty :-).  Lots of people just like to read and not post, and there is
absolutely nothing wrong with that.  Lack of response does not translate
to an affirmative statement.

>Sounds like I could just use `dd' in a loop using the block size and
>Alternating drives, to reconstruct a real, contiguous image somewhere
>(probably onto other tapes).  ...

But you'd have to do it one block at a time.  With System-V tape
semantics, every time dd finished copying one block, the tape would
automatically advance to EOF.  This can be dealt with, but, as I said,
it will take some relatively advanced shell programming (although not
difficult in something like Perl).

John R. Jackson, Technical Software Specialist, jrj AT purdue DOT edu