Re: Are their any known problems with using "rewrittable CD's" for backups?

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From: _Vanguard_ (lh_news_at_yahoo.com)
Date: 12/29/04 

Date: Tue, 28 Dec 2004 20:52:34 -0600

"Dev" <noreply@noreply.com> wrote in message
news:%23cGmuRU7EHA.2552@TK2MSFTNGP09.phx.gbl...
> Sorry if this is off topic, but does anybody know of any known
> problems with regard to reliability when using rewrittable CD's as
> compared to not rewrittable CD's for use as backing up important
> files?

If using UDF (i.e., packet writing), like with DirectCD or InCD, and
when using rewritable CDs (i.e., CD-RW), then a fixed amount of it gets
eaten up for the file table. This allows the file table to be
overwritten as there are changes to files on the CD-RW, just like on a
floppy. That means you immediately discard a portion of the CD-RWs
capacity due to the file table which might end up with lots of unused
space within it. With continuous track recording (versus packet
writing) and CD-R, the table is variable sized so you waste less space
(but each session has another TOC so their physical consumption is
cumulative).

When it comes time to reuse the CD-RW backup media, you will waste a
long time erasing them (if using continuous track recording instead of
UDF). Is it worth saving the pennies on the CD-Rs that you would toss
to use new ones versus the worth of your time waiting around to erase
all those CD-RWs? You lose 17 cents to discard the CD-R or you lose 20
minutes of your time to erase the CD-RW. Not a big deal (of losing
time) if there are only a couple of discs involved but it is a big deal
when you have lots of them to erase. If you use UDF to eliminate the
need for erasing the disc as would be required for multisessioned discs
with continuous track recording, you sacrifice compatibility. Not all
operating systems come with a built-in UDF reader (and even fewer with a
UDF writer). Even Windows XP only comes with continuous track recording
mode and you need to get DirectCD or InCD if you want UDF writing
(that's why WinXP's CD burning software has to buffer the files on the
hard disk before writing them on the CD-R[W]).

CD-RW media only supports a limited number of erasures, like 1000.
There is no alarm that alerts the user that they have hit the
recommended maximum erasures, so the disc becomes increasingly flaky.
Unless you record the number of erasures on each CD-RW disc, how will
you know when to discard it? Are you going to wait until you need those
critical files and then find you cannot read them from the CD-RW disc?
By the way, make sure you enable the verify option in your backup
software since an unverified backup is worthless is you find out later
that you cannot read from it.

If you don't use UDF (packet writing) for CD-RWs, then the TOC is much
smaller and you have more space but then subsequent writes will create
another session (i.e., you end up with a multi-session disc). That
means you have old deleted files no longer accessible via the new TOC in
the latest session but they still physically occupy space on the disc.
You end up with discs with ever decreasing capacity -- until you spend
the time to erase them. A 20-disc backup set of CD-RW media would take
hours to erase before you could reuse them for the next backup. I'd
rather lose the $3 worth of CD-Rs than waste 6 hours erasing CD-RW
media. I don't have a bank of twenty CD-RW drives to erase 20 CD-RW
discs in parallel. I have only one CD-RW (per host) and I have to erase
the CD-RW discs sequentially.

It may also take longer to save your backups onto CD-RW than to CD-R.
Depending on your CD-RW drive, you'll notice the write speed for CD-R is
much higher than the write speed for CD-RW. My CD-RW lets me write CD-R
four times faster (or in one-fourth the time) as it will for CD-RW.
However, your backup software my itself restrict what is the maximum
write speed for compatibility with older drives so there may be little
or no speed difference.

CD-RW is still much more expensive than CD-R media. I can get a
100-disc package of CD-R for $17 and a 100-pack of CD-RW for $40. 17
cents versus 40 cents? I can go through 2.4 CD-R discs before I've used
a CD-RW just once. Not much of a difference until you account for the
greater time to write and especially to erase CD-RW. If you consider
your time of no value then CD-RW - over time - will be cheaper. Also,
CD-RW media varies regarding compatibility and reliability depending on
the drive. So the CD-RW media that you have tested is reliable with
your CD-RW drive may not be reliable when read using a different CD-RW
drive, like when restoring to a different computer or having to replace
your CD-RW drive. You will also place twice as much wear on your CD-RW
drive to use CD-RW media for backups than when using CD-R. With CD-R,
you write once per disc. With CD-RW (and when using continuous track
recording), you write twice, once to erase and again to actually write
files to it.

I use CD-R for backups (full and differential) and for drive images, and
a hard drive for daily backups (incremental). Eventually I'll have to
move to DVD-+R because of the aggregate size of all my files. I might
get a USB drive for backup but it would only be for temporary backup,
like around a week, since I wouldn't trust my backup on the reliance of
the mechanicals of a hard disk: the data on the platters might be okay
but the drive's mechanicals fail due to surge, static, shock, defect, or
wear. I like backup media that is independent of any drive mechanicals
so I could always replace the drive and still have my non-temporary
backups. If you feel compelled to use CD-RW media for backups, just use
it for the incremental daily backups and use CD-R for the weekly,
monthly, and any full backups.

Personally I don't see the advantage of using CD-RW media due to the
hassle of having to erase them and the waste of my time to do so. I
won't use UDF (packet writing) because I want reliability and
compatibility, plus I have found the UDF writers (which load as
installable file systems), like DirectCD and InCD, to be flaky and
generate blue screens too often. If they truly are to be used for
backups then you aren't going to be overwriting them anyway since, well,
they are part of your backup set. So why waste the extra money on
slower-writing media for a feature that you won't use? For daily
incremental or other temporary backups, using CD-RW or hard disks makes
sense but not for full or differential backups and not for drive images
(needed for disaster recovery where you want the more reliable and more
compatible CD-R media).

For the option to rewrite, you get slower-writing media, perhaps twice
the wear on your drive (unless you go with the less compatible UDF with
its less stable installable file system drivers), and the long time to
erase (so you have full capacity again if using continuous track
recording for multisession for compatibility instead of UDF packet
writing with less compatibility). It's an okay tradeoff for short-term
backups (which don't incur more than 1000 erasures per CD-RW disc) but
not for long-term backups or for disaster recovery. 

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