[H-GEN] Tape Drives

Michael Anthon michael at anthon.net
Mon Jan 7 06:28:36 EST 2002


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----- Original Message -----
From: "Patrick Nichols" <pat at humbug.org.au>
<snip>
> As for prices and such, a sony dds4 internal drive is about 1700-1800
(lots
> of places), and imation dds4 tapes are 43.60 each or 42.60 for 10
> (datarec.com.au - gotta love google),  They sell imation dds3 tapes for
> 19.10 each for a box of 10.  Add a cheapo scsi card and 50-68 pin adapter
> and I'm in business.  Depending on how tight they are I could save some
> money by buying some dds3 tapes to go with the drive, and having a mixture
> of tapes.
>
> The reason I think that a dds4 drive will be a better solution than the
ide
> one proposed earlier is that I can't find anywhere that sells the tapes
> online, they seem to have very poor market infiltration with their 30/60
> drive, and I would expect the tapes to be really expensive.  Of course
after
> looking at what is available I would like to get a dlt drive and tapes,
but
> I think that is a little more money than they are willing to spend.  Plus
> dds4 is a tried and tested technology.

Keep in mind that depending on the type of data being stored, you *may* get
somewhat more or less than the 40G onto a DDS4.  The 40G is essentially
double the raw capacity of the tape and assumes a compression ratio of 50%
when using the drive's internal compression algorithm.  In practice I find
that the compression ratio is generally somewhat better than that [1].  You
will find that if your data is largely textual in nature then it will
compress rather nicely.  DLT is definitely better both in capacity and
speed, however you will pay for that improvement, as you have discovered.

The other method I have used is to use something like 'tar c stuff | gzip -9
| dd of=/dev/tape'.  It's a little heavy on the CPU usage, but in the middle
of the night, who cares?  The main reason I did this instead of using the
drive compression was that the version of tar on the system in question had
problems handling a few largish files.

To add another thought into the tape vs. HDD argument... if you have to back
up a database, quite often the database needs to be taken offline if you
wish to make a backup of it's data files.  We use Oracle at work and with
that it is possible to place the database into a mode where you can take a
consistent backup of the datafiles while the system is still active.  While
in this mode certain things don't happen as usual and large volumes of
transactions will cause problems.  As a result it's imperative that the
actual copy of the files is made as quickly as possible.  To this end we
have another large drive in the system that the backup is made to, then the
database is switched back to the normal mode and the copied files are
written to tape and then compressed on the drive.  This way I can keep a
week or so of backups online so that if the day ever comes that someone does
a Bad Thing to the data (as opposed to a system or hardware failure) we do
not need to wait for the latest tapes to be retrieved from the offsite
storage.  Oh, btw, on this system the actual file copy takes about 30
minutes, the write to tape about 3 hours and the final compression about 6
hours.... and I also do a test on the tape to make sure it's readable and
that takes another 3 hours or so..... I often get a call in the afternoon
from the person changing tapes "The backup has locked up and the tape hasn't
ejected, can you reset the machine please? [3]".  Perhaps I should start the
backup earlier 8^)

Hmm.. waffle!

Oh... one more thing.  Depending on your backup scheme and the number of
systems you are backing up, media costs can be quite excessive.  Our IT
people recently changed the scheme they used.  Originally we had 16 tapes on
a 4 weekly rotation for the first 4 days of the week and then a permanent
backup was taken on a new tape every Friday.  We now have 19 tapes in a 4
week rotation with a permanent backup every 4 weeks.  So we have gone from
52 news tapes (plus replacements for wear and tear on the rotation set) per
year to 13 new tapes per year (most of these are DLTs with a few DATs).
This has netted the company a saving of over $500,000.00 per year
nationally.

Cheers
Michael

[1]  Because I seem to be able to fit a lot more than I should onto DDS3
tapes. [2]
[2] As an aside, does anyone have any clue as to how to determine the
remaining capacity on a tape?  It's something that's bugged me for a while
and I can't find an answer to it.
[3] Erm, this is an E250 running Solaris, most of the boxes there, apart
from the ones I look after, are running Novell or NT and I have given up
explaining that it's not locked up, it just hasn't finished yet.  I normally
just kill the verify and eject the tape, seems to keep them happy 8^)


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