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What's so great about tape?



 
 
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  #11  
Old September 4th 03, 07:01 AM
Rob Turk
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"Paul Rubin" wrote in message
...
Malcolm Weir writes:
Still, what you've experienced shows the benefit of writing lots of
file marks. Classic Unix (and Unix-like) things (tar, etc.) want to
write *one* tape file, and if part of that file is corrupted, then
it's all lost. But if you write a filemark every (say) 100MB, then
the chances of being able to seek past the bad spot improve
dramatically.


I've heard that if you write two consecutive EOF marks on a DDS or 8mm
tape, that's interpreted as end-of-tape and there's absolutely no way
to read past it. It's not like the old days of 9 track tape.


One more example of half-truth. Filemarks are logical entities on tape, just
like data blocks are. Reading past two filemarks is just as well possible as
reading past any other data entity on tape. You can start your tape write
with 500 filemarks, then a single block of data. You'll always be able to
get that single block back.

The story of not being able to read past two filemarks is based on the fact
that it normally marks the end of a write session. In many data streams, the
end of *logical* data is marked by writing two consecutive filemarks. It
serves as an append point for later sessions. Tape, being sequential media,
also writes a *physical* end-of-data marker at that point, and usually it's
followed by an erase gap. Depending on the implementation, the erase gap is
a single track, or can be a number of tracks wide. It invalidates any tracks
behind the end of data marker, by partially or completely erasing them.

Reading past such an area is normally prevented in firmware, as anything
behind the EOD marker is not considered part of the current tape sequence
(remember, sequential data). However, most drives can be persuaded to skip
such an erase area to the next valid filemark, and can continue to read from
there on. Data recovery centers use this feature to get 'overwritten' data
back. I've written software to even automate the recovery process on
EXB-8200 through 8505XL, it's very easy to do. On most helical scan drives
you can do this. On most linear drives you can do this too (except DLT7000,
as it destroys multiple tracks with a single write).

Rob


  #12  
Old September 4th 03, 09:15 AM
Marcin Dobrucki
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Rob Turk wrote:
"Marcin Dobrucki" wrote in message
...

idunno wrote:

I think I'm missing something, but why do people use tape for backups?
It seems very costly and slow.


August 2003 issue of SysAdmin Magazine (www.sysadminmag.com) has an
article just for you. "Tapes: A Modern History, Trends", by Henry
Newman, p. 43

This article is so full of semi-technical nonsense that it ain't funny
anymore. I agree with the overall conclusion (tape is here to stay) but this
guy has definitely not been doing his homework on helical scan recording.
This stuff is directly copied out of 10-year old Quantum DLT sales pitches.
Yuck...


I'll take your word for it since I don't know much about the stuff,
but I thought the article pointed out a few good things about the
original question, i.e. what's great about tape. And like you, I agree
with the conclusion, I can't see it go away anytime soon.

Out of curiosity, can you point me at a good reference about the
recording technologies? I need to brush up on this subject.

/Marcin

  #14  
Old September 4th 03, 02:24 PM
paul blitz
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There are several reason for doing backups of any kind. The main two seem to
be:

1) recovery in case of a drive death
2) archiving, to let you recover deleted information, take snapshots etc

The first of these issues can, to a certain extent, be covered by having a
disk raid system.... but even that doesn't solve the disater situation
(fire, flood etc)

The second issue is a way of either allowing you to recover accidentally
changed / deleted data, as well as allowing you to "snap-shot" your data (as
is often needed for legal reasons), and that sort of thing.

Of course, there are 2 aspects of the backup: "protecting" your operating
system & software, and "protecting" your data... its all very well having a
great data backup, that lets you recover al your data in 2 hours flat, if
(because of bad documentation or whatever) it takes you 3 days to correctly
re-install the underlying software!!!!!

From what I have seen (we have recently intslled a tape system that uses
ArcServe) tape backup has NOT really improved in the last 15 years: the
software is still very slow, very cumbersome, and still not easy & obvious
to use.

The next thing to ask is "what alternatives to tape are there": and as
others have said, there is only ONE that has enough capacity: hard disks!
CDR's are ok for small amounts of data (eg data on a personal computer,
rather than a server), but are nowhere near big enough or "good enough" for
(longer-term) backups of large amounts of data.

As well as needing backup where I work, I am involved with a hospital radio
station, where we have a computer playout system: the work needed to recover
all the music an associated data is huge, so it HAS to be backed up: in this
case, both PCs have a small HD with the OS & software on (which we have
ghosted onto CD, and can rebuild the pc in about 10 mins flat!), and a big
HD for the music in a caddy. One of the PC's also has a spare caddy, where
we plug in a second hd & copy all data to it: that HD is kept off-site. In
this case, we have no need for keeping archives.


I think there is still very much a place for tapes, mainly for longer-term
data archiving of large amounts of data.


Paul





"idunno" wrote in message
...
I think I'm missing something, but why do people use tape for backups?
It seems very costly and slow.

I can imagine two benefits from tape, though. The backup cartridge is
not nearly as fragile as say a hard disk and is therefore better to
transport offsite. Some better tape formats and tape libraries can
backup vast quantities of data easily and unattended.

I've been using hard disks in removable racks & external cases as well
as DVD-R. I've had bad experiences with travan a few years ago and
I've hear bad things about DAT reliability. I've been thinking about
implementing VXA or DLT based backup in a small office, but I'm having
a difficult time justifying the cost. These drives and the tapes are
quite pricey esp. if these tapes can only be used a limited number of
times. I am very worried about various nuisances with the media or
drive heads resulting from daily backups.

Sorry if my newbiness is showing. What am I missing here? How
exactly are the costs justified? For whom is tape backup best suited?



  #15  
Old September 4th 03, 03:32 PM
Paul Rubin
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"Rob Turk" writes:
I've heard that if you write two consecutive EOF marks on a DDS or 8mm
tape, that's interpreted as end-of-tape and there's absolutely no way
to read past it. It's not like the old days of 9 track tape.


One more example of half-truth. ....
Reading past such an area is normally prevented in firmware, as anything
behind the EOD marker is not considered part of the current tape sequence
(remember, sequential data). However, most drives can be persuaded to skip
such an erase area to the next valid filemark, and can continue to read from
there on. Data recovery centers use this feature to get 'overwritten' data
back. I've written software to even automate the recovery process on
EXB-8200 through 8505XL,


The subject has come up here on c.a.s. or comp.periphs.scsi a few
times in the past and there don't seem to be any such methods
available to ordinary users. There was some kludge posted about
starting a read and powering down the drive at some critical instant
and powering it up again, or something like that, which sometimes
confuses the drive enough to read the data. Anyway, yes, I shouldn't
have said "absolutely no way" since it has to be possible if you can
modify the firmware. However, if it requires firmware mods or the
resources of a data recovery center, that's close enough to impossible
for most users.
  #16  
Old September 4th 03, 04:40 PM
Boll Weevil
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On 3 Sep 2003 14:55:07 -0500, Eric Lee Green wrote:

In article , Boll Weevil ruminated:
First of all, nobody makes a robotic hard drive changer. We go through
about 500 tapes a day using 20 to 40 tape drives concurrently all
managed by automated robotic tape libraries. I can't imagine to trying


Let me get this straight. You back up 50 terabytes per day? Or are you
using older/smaller capacity technology, let's say DLT1, and backing up,
say, 10 terabytes per day?

10 terabytes per day = 3650 terabytes per year. You're saying that your
installation is pushing 3650 terabytes of data per year through your
systems? Or are you saying that, due to the inefficiencies of current
tape backup solutions (which operate upon a whole-file basis rather
than on a differential block basis), you need 3650 terabytes of tape storage
to store, say, 365 terabytes of changed data?



See if you can add this up. To start, we have about 200 Sun servers and
about 1200 NT servers. About 100 Sun servers and about 100 NT servers
are on the SAN and share the following EMC and Hitachi subsystems:

7 EMC 8830 frames with about 13 TB raid 10 useable, each
1 Hitachi 9980V frame with about 45 TB raid 5 useable

There are a whole lot of direct attached SCSI disk arrays and internal
disks in each of the 1200 NT servers. I can't even start as to how much
storage these servers account for. These all get backed up.

We use STK 9840A and 9840B drives. These drives can do about 20GB per
tape at about 20 to 40 mb per second.

500 tapes??? Well, I think I've mistaken or I kicked out an old number.
We probably use twice that. Most of the SAN disk is used for Oracle and
SAS databases. Oh yea, we also have about 200 TB of mainframe but IBM
manages that. So, do some math and figure out how many tapes we use on
a full backup. You can save me some time.
  #17  
Old September 4th 03, 07:05 PM
Rob Turk
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"Paul Rubin" wrote in message
...
"Rob Turk" writes:
I've heard that if you write two consecutive EOF marks on a DDS or 8mm
tape, that's interpreted as end-of-tape and there's absolutely no way
to read past it. It's not like the old days of 9 track tape.


One more example of half-truth. ....
Reading past such an area is normally prevented in firmware, as anything
behind the EOD marker is not considered part of the current tape

sequence
(remember, sequential data). However, most drives can be persuaded to

skip
such an erase area to the next valid filemark, and can continue to read

from
there on. Data recovery centers use this feature to get 'overwritten'

data
back. I've written software to even automate the recovery process on
EXB-8200 through 8505XL,


The subject has come up here on c.a.s. or comp.periphs.scsi a few
times in the past and there don't seem to be any such methods
available to ordinary users. There was some kludge posted about
starting a read and powering down the drive at some critical instant
and powering it up again, or something like that, which sometimes
confuses the drive enough to read the data. Anyway, yes, I shouldn't
have said "absolutely no way" since it has to be possible if you can
modify the firmware. However, if it requires firmware mods or the
resources of a data recovery center, that's close enough to impossible
for most users.


What you describe was for 4mm DAT drives, start a write and power down, so
no EOD marker would be recorded. Then spacing over the (now corrupted) erase
area would work.

For Exabyte drives, for 8200's no mods were needed. The firmware (32KB of
code on an 8051 CPU) did not require any changes to space to the next
filemark after EOD. Not enought smarts to detect EOD while spacing ;-) For
all other 85xx drives a standard EE-image implementing 'directory support'
allowed you to do the same thing. No tricks, no switching power, just using
the features of the drive.

Rob


  #18  
Old September 4th 03, 07:10 PM
Rob Turk
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"Boll Weevil" wrote in message
...
On 3 Sep 2003 14:55:07 -0500, Eric Lee Green wrote:


See if you can add this up. To start, we have about 200 Sun servers and
about 1200 NT servers. About 100 Sun servers and about 100 NT servers
are on the SAN and share the following EMC and Hitachi subsystems:

7 EMC 8830 frames with about 13 TB raid 10 useable, each
1 Hitachi 9980V frame with about 45 TB raid 5 useable

There are a whole lot of direct attached SCSI disk arrays and internal
disks in each of the 1200 NT servers. I can't even start as to how much
storage these servers account for. These all get backed up.

We use STK 9840A and 9840B drives. These drives can do about 20GB per
tape at about 20 to 40 mb per second.

500 tapes??? Well, I think I've mistaken or I kicked out an old number.
We probably use twice that. Most of the SAN disk is used for Oracle and
SAS databases. Oh yea, we also have about 200 TB of mainframe but IBM
manages that. So, do some math and figure out how many tapes we use on
a full backup. You can save me some time.


So you work for Boeing, right? One of the largest Powderhorn sites I know
of...

Rob


  #19  
Old September 4th 03, 07:52 PM
Paul Rubin
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"Rob Turk" writes:
For Exabyte drives, for 8200's no mods were needed. The firmware (32KB of
code on an 8051 CPU) did not require any changes to space to the next
filemark after EOD. Not enought smarts to detect EOD while spacing ;-) For
all other 85xx drives a standard EE-image implementing 'directory support'
allowed you to do the same thing. No tricks, no switching power, just using
the features of the drive.


Thanks, that helps. How is the situation for current Exabyte/Ecrix drives?

Also, suppose you have an M3 or VXA2 cartridge and cut the tape
somewhere in the middle with a scissors. Can you get back (most of)
the data from both pieces without having to go too crazy with special
programming? Is there enough redundancy to not lose data around where
the cut is? I figure you can't splice the tape, but maybe you can
spool each piece onto another cartridge or something.
  #20  
Old September 4th 03, 08:10 PM
Dan Foster
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In article , Boll Weevil wrote:

See if you can add this up. To start, we have about 200 Sun servers and
about 1200 NT servers. About 100 Sun servers and about 100 NT servers
are on the SAN and share the following EMC and Hitachi subsystems:

7 EMC 8830 frames with about 13 TB raid 10 useable, each
1 Hitachi 9980V frame with about 45 TB raid 5 useable

There are a whole lot of direct attached SCSI disk arrays and internal
disks in each of the 1200 NT servers. I can't even start as to how much
storage these servers account for. These all get backed up.


Rough calculations shows that using hardware compression for a LTO-2 setup,
one could do all of the above in about 2 (or so) fully decked out IBM 3584
LTO libraries (just as an example), assuming an average size of directly
attached storage for each of the NT servers being 1 TB. A decked-out LTO-2
library with 6 frames should yield in the neighborhood of about 720 TB of
tape storage capabilities.

If 1200 servers * 1 TB = 1200 TB; that'd be one decked out LTO library and
a second library with about 240 TB of available tape space. For the other
stuff... 13 * 7 = 91 plus 45 TB = 136 TB. So you'd still have 104 TB of
free data space, and capable of doing a single full backup for everything
with two libraries and about 3500 tapes.

This assumes 400 GB (hw compressed) LTO-2 tapes; if you are using 20 GB
tapes in uncompressed mode, then your tape requirements goes up by 20 times
3500 for at least 70,000 tapes. Also, if the average per-NT server for
storage is other than 1 TB, that would also influence number of tapes
required, as well.

-Dan
 




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