Read-Only RAID-5 on DVD? :-)

[James Gray]

On 3/7/06 9:01 AM, "Harijs Buss" wrote:

> The lifetime of DVD+-R disks is probably even shorter than that of CD-R.
> 
> Thinking about backups written on DVD+-R, should be logical to use the same
> technology as RAID-5: for example, write 4 DVD disks containing as much
> information as usually 3, but with redundant information which would allow to
> restore all info even if one (any) of these DVD's become unreadable or has
> been lost. 
> 
> For privacy lovers, these 4 (or more) DVD's could be stored each at different
> location; only when somebody has been able to collect at least 3 of them he
> would have the information available :-)  Certainly, 4 was mentioned only as
> example, this might be any number starting from 3 or more (like hard disks in
> RAID-5). 
> 
> As it happens with all good ideas, probably somebody has already thought about
> that and made some software to burn such read-only DVD RAID-5  :-)  Please
> help me with link to existing software.

So if you loose a disk from the "array" which is read-only...how the hell do
you replace and rebuild the dead disk?  RAID-5 has distributed parity,
meaning the parity would have to be written as the array is written to.
This is non-linear for any given disk in the array and thus not possible
with ISO9660 (which requires a constant stream of data...ie linear data
writes).  If you want redundancy, just burn multiple copies of the same
disk.  If you need more space than DVD's offer (single or dual layer) maybe
look at a real backup solution with LTO/AIT/VTL/etc or plain old
disk-to-disk backups.

Hard drives are cheap, reliable and reasonably robust these days (at least,
if not more, robust than tapes).  Just buy a pair of 300GB drive for weekly
backups and a couple (4) smaller ones for incrementals during the week.
Like this:
Week 1 Sun:      300GB Drive #1
       Mon-Fri:  100GB Drives 1-5 (incrementals since last full backup)
Week 2 Sun:      300GB Drive #2
       Mon-Fri:  100GB Drives 1-5 (incrementals since last full backup)
...rinse and repeat :)

 Provided you rotate the two 300GB drives one week about, the worst you'd
ever have is two weeks of data gone.  Most SME's can recover from that far
cheaper than backup solution to afford higher availability would cost.  If
you had two sets of drives for the incrementals, then you'd only loose one
week.

Backup is very much a matter of "how much data can you afford to loose, how
much can you afford to backup" and the sweet spot that answers those two
questions is different for almost every person/company.

HTH,

James