[Techtalk] formatting half a mirrored raid...

Maria Blackmore mariab at cats.meow.at
Fri Jun 13 19:11:56 EST 2003

On Fri, 13 Jun 2003, Walt wrote:

> Okay, so I did this and now lsraid for /dev/md0
> shows this:
>    [dev 9, 0] /dev/md0   ####bunch of numbers#### online
>    [dev 3, 1] /dev/hda1  ####bunch of numbers#### good
>    [dev ?, ?] (unknown)  ####bunch of numbers#### missing
>    [dev 22, 65] /dev/hdd1 ####bunch of numbers#### spare

hmm, that's ... interesting

can you post the contents of your raidtab for md0 please?

> Also, I would expect the system to start
> working considerably to copy over the 110 gb
> of data, but it's doing nothing.

For some reason, it appears to think it's a hotspare, which is a bit
weird.  I'll know more when you paste the contents of the raidtab :)

> Yes, I am pondering this decision... Each 73 gb
> drive costs c. $450... So adding a parity drive is
> fairly expensive, and I need all the space I can
> get. We eating up about 10-15 gb a month +/-.

You may wish to consider an external storage array that uses u160 SCSI as
the bus protocol to communicate with the host computer, but you stack it
full of cheap, commodity IDE drives.  The idea being that because you have
hot spares online and ready to go, and would be using RAID5, the loss of a
less reliable IDE drive will not be harmful, and can be quickly and
automatically rebuilt on the spare drive.  The loss of an individual drive
will not harm anything else, since all the caddies are hotswappable, and
each drive is on a seperate IDE bus.

It's not 100% foolproof, and it's not the fastest thing in the world,
probably topping out at 140 MB/sec or so, but it should be good for you.

Failing that, I have a friend who's selling a pair of EMC storage arrays,
good for 18 TB each. :)

> parity drive and switch to another raid level at a
> later day? Or now that I have it setup as raid0,
> I'd have to start over with blank drives to switch
> to level 3 and copy the data all over again?

In theory, it might be bodgable, I wouldn't recommend it.  It's always
best to start from blank, and restore from a backup.

> Linear? "And he learned *new* things to say!"
> How do I make it linear? Arg! Is that another
> raid level or just a setting for level 0 raid?

It's another raid level, though you may wish to consider using LVM to form
a linear array, and then use an extensible filesystem such as XFS.

I always advocate the use of backups, in which case a small DLT changer
would probably work out quite well for you, though there are better
technologies starting to appear nowadays.  A single DLT cartridge should
be good for 40 GB of uncompressed data.

Lastly, yet another thing to think about would be combining the best of
SCSI and IDE.  SCSI for reliability and speed and usefulness, and IDE for
cheapness.  The cost of a 200 GB drive and an IDE<>SCSI convertor should
still be cheaper than a 73 GB native SCSI drive.  Something to think, i
think :)  (ymmv)


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