Re: [9fans] mass storage jukebox

[Geoff Collyer <geoff@collyer.net>]

RAID sounds like an appealling alternative to magneto-optical at
first.  But if you bought all the disks in your RAID array at the same
time, they're all going to fail at about the same time, for
sufficiently fuzzy values of ``about''.  If you keep one hot spare, as
seems to be typical, when one disk dies, you need to replace it before
a second one dies.  There's also the possibility of multiple
concurrent failures to due earthquakes, something being dropped on the
disk array, nearby lightning strike, kittens pulling the power cord
out of the wall with their teeth, etc.  But if you're on two weeks'
vacation (or four weeks' if you work somewhere other than the US), you
may not get back home in time to replace the dead disk before a second
one dies.  You might, but I'm paranoid and I've been collecting files
for 30 years, and I'd rather not lose them.  Also during those 30
years, I've seen and heard an awful lot of magnetic disk head crashes.
I really, really like the lack of optical disk head crashes.

I also like automatic backups that don't involve me doing anything.
Once DVD-RAM jukeboxes exist and drop way, way down in price, they'd
be a reasonable alternative to M-O disks, though DVD-RAM disks still
aren't write-once.  I don't think any of the current contenders for
DVD standard are write-once and yet allow incremental writing without
major penalty (megabytes thrown away); true?  The DVD standards
mud-wrestling / squabbles / wars don't help, but everybody wants a
royalty.  Philips apparently made a lot of money off the Compact
Cassette and the CD.

As for jukebox compatibility, if you have manuals, see if they mention
SCSI-2 MMC (multi-media command set) support.  There's an MMC subset
for jukeboxes.  If you can't tell, plug the jukebox into a file server
and see if it's recognised when you boot the file server and configure
it to expect the jukebox.  The first device in the config string after
"j" is the robot arm and the rest are M-O drives inside the jukebox.
I think failure to recognise the jukebox will produce a flurry of SCSI
error messages.

For optical disks, you want magneto-optical disks that match the
drives in your jukebox.  HP drives can write one generation back and
read two generations back.  The usual sizes are 1.3, 2.6, 5.2 and I
think 9.4 GB (I don't have any of the 9.x GB drives).  There may have
been a 650MB size in antiquity.  The disks are standardised by ISO and
the standards can be found on www.iso.ch, though you probably have to
pay to read them.  At least you should be able to get the numbers of
the relevant standards from the ISO web site.  Note too that sector
size is a function of total size.  For 2.6GB disks, sectors are 1024
bytes; for 5.2GB disks, sectors are 2048 bytes.  I suspect that 9.xGB
disks use 4096-byte sectors.  Make sure that your file server block
size (RBUFSIZE) is at least as big as your M-O disk sector size.  This
probably won't be an issue for most people until at least the next
generation of M-O disks, assuming that they use 8192-byte sectors.