[lug] writeable CD curiosity

[Paul E Condon]

D. Stimits wrote:

> Thinking about firewalls that run on read-only media, but need logging 
> ability, solid state ram devices seem ideal for reducing 
> size/power/failure rate associated with devices that have fans (the 
> typical PC used for firewalling/bridging). All of this of course made 
> real by my current hard drive failure that was on /var/ partition.
>
> So I am curious about limitations of current writeable CD hardware. 
> Initial limitations of ISO9660 filesystems come to mind, but I'm not 
> actually interested in ISO9660 filesystems. I am wondering what in the 
> hardware would prevent a rewriteable CD from being used with ext2 or 
> other "ordinary" random access filesystem? Performance would be 
> incredibly bad, but for logs I wouldn't care anyway.
>
> As a half-version of a fully rewriteable CD filesystem partition, I'd 
> consider something from recent posts: ram-based partitions, which 
> would occasionally append to a CD-RW or CD-R either when it reaches a 
> certain capacity or when logs are rotated, e.g., log to ram, and then 
> over time transfer from ram to CD-R or CD-RW; pop the logs out via 
> ejecting the CD, and put a new one in, whenever the 650 MB or so of 
> logs are reached. A full historical log record that can be browsed and 
> analyzed.
>
> Another possibility that I wonder about only for curiosity, and not 
> necessarily for logging, is the concept of a RAID 1 where the 
> partitions being mirrored are (a) a ram-based filesystem and (b) a 
> rewriteable CD system...if power fails and ram is flushed, then the CD 
> is used for rebuilding the array; or while power runs, the CD could be 
> ejected and saved, a new CD added, and have it automatically rebuild 
> the array via copy from ram to CD-RW. What are the hardware 
> limitations to this?
>
I think there is a useful discussion of these issues in the man page of 
growisofs. ext2 does an
update of a single node on disk each time there is a write to (or read 
from) a file. This rapidly
degrades the rewriteable storage medium. The update on read can be 
eliminated by marking
the device read-only. The rest is more complicated than I can recall.