Help on Tape Marks

[Shiera]

Hello everyone,

I really need your help on the following.

Excerpt from SCSI Stream Commands-2 (SSC-2) Working Draft:
1.) "Filemarks are special recorded logical objects not containing
user data."
2.) "Setmarks are optional special recorded logical objects not
containing user data."

QUESTIONS:
What really are filemarks and setmarks? I assume these are
multiple-byte logical objects. But since these do not contain user
data, then what are the contents of these tape marks? Can these be
modified by the application client?

What's the size of filemarks and setmarks? Can their size be modified
by the application client?

Thank you so much for your help!

- shiera -

 

[Rob Turk]

Hello everyone,

I really need your help on the following.

Excerpt from SCSI Stream Commands-2 (SSC-2) Working Draft:
1.) "Filemarks are special recorded logical objects not containing
user data."
2.) "Setmarks are optional special recorded logical objects not
containing user data."

QUESTIONS:
What really are filemarks and setmarks? I assume these are
multiple-byte logical objects. But since these do not contain user
data, then what are the contents of these tape marks? Can these be
modified by the application client?

What's the size of filemarks and setmarks? Can their size be modified
by the application client?

Thank you so much for your help!

- shiera -

Filemarks and setmarks are just flags on tape that are used to mark certain
sections on tape. They allow you to create a logical format on tape. On
harddisks it's easy to record that file XYZ starts on track 200, head 3,
sector 12. Tape is a sequential medium, so it's a lot harder to find
specific locations. To prevent that you have to read the entire content from
the beginning each time you need to find data, you can use filemarks and
setmarks. Typical data lay-outs on a non-partitioned tape could be:

BOT-HHH-F-F-EOD (Tape just labelled)
BOT-HHH-F-DDD-F-III-F-F-EOD (Tape with a single backup set)
BOT-HHH-F-DDD-S-DDD-S-DDD-S-DDD-F-III-F-F-EOD (Multiple savesets in a single
session)
BOT = Begin of tape
H = Header information (readable by the system)
F = Filemark (Searchable, but not readable)
D = Data (readable by system)
S = Setmark (Searchable, but not readable)
I = Index information (readable by system)
EOD = End of Data (blank tape)

The header information is written to tape when you 'label' your tape from
the backup application. The system will write a couple of blocks and then
two filemarks. EOD is automatically recorded, the system doesn't tell the
drive to.

When reading back, the system will first send a "Space to End of Data" to
find the last recorded entity. It will then send a "Space 3 filemarks back".
If the tape only contains a label, the drive will report it's back at BOT.
If there are savesets on the tape, the drive will be at the beginning of the
filemark just prior to the Index data. So a "Space 1 Filemark Forward" will
get the system to the index data.

The system reads the index data, which shows that file XYZ starts at saveset
2, block 15. The system sends a "Rewind", then a "Space 1 Filemark Forward".
The drive is now at the begin of user data. The system now sends a "Space 1
Setmark Forward", finding the location of the second saveset. Then a "Space
15 blocks forward" will get the drive to the start of data blocks belonging
to file XYZ.

Long story, but hopefully it illustrates how filemarks and setmarks are
used. As they are internal markers to the tape drive, their size depends on
the tape technology in use. Some tape streamers know long and short
filemarks, some can be modified in length. Most of the client apps don't
care, they use them as signposts. Which is what they are designed for...

Rob

 

[Malcolm Weir]

Hello everyone,

I really need your help on the following.

Excerpt from SCSI Stream Commands-2 (SSC-2) Working Draft:
1.) "Filemarks are special recorded logical objects not containing
user data."
2.) "Setmarks are optional special recorded logical objects not
containing user data."

QUESTIONS:
What really are filemarks and setmarks? I assume these are
multiple-byte logical objects. But since these do not contain user
data, then what are the contents of these tape marks? Can these be
modified by the application client?

Rob covers most of it very well, but with a slant towards a particular
family of products(!)

But a couple of notes: since filemarks and setmarks do not contain
user data, they cannot be modified by the application; that's the
meaning of "not containing user data"!

What's the size of filemarks and setmarks? Can their size be modified
by the application client?

Again, no. They are just markers. In some implementations, they
don't even exist in the same data area of the tape as the user data;
they can be written in a "parallel" recording area so that they occupy
no space in the user recording at all!

Some tape recording systems have distinct data and system recording
areas; the system area can be used for things like checksums, bad tape
indications (like bad blocks on disks), and positioning information
such as the file and/or block number.... and, of course, file or set
marks. This is useful since in high-speed searches the tape drive
doesn't have to wade through the dense user data, merely the system
data. (Example: US Patents #843995 and #843740)

In other systems, it may help to remember that the bit pattern the
application sends to the drive is almost always *not* the bit pattern
recorded on tape; there are both encoding schemes (NRZI, etc.) and
additional error detection/correction mechanisms that "distort" the
user data. So it is perfectly possible to create a bit pattern on
tape that *cannot* be created by user data. It is these patterns that
can be used to indicate an FM or SM. In these cases, the marks *can*
occupy tape space.

The last mark that you may be interested in is the End of Recording
mark, EOR; this is *usually* a larger length of tape with a specific
pattern recorded on it, so to be easy to locate under all
circumstances.

- shiera -

Malc.