MTN backup¶
Some QIC-80 tapes carry the letters MTN in their volume table entries. The
software that wrote them is not identified; MTN may be Mountain Network
Solutions' backup software, but that is not confirmed, so the profile is
named after the bytes. These tapes hold QIC-113 Basic DOS volumes compressed
with QIC-122, with a volume table layout and extent width that differ from
the standards. Every offset on this page was read off real tapes, not taken
from a specification.
Profile: mtn (packages/qiclib/qiclib/volume/profiles/mtn.toml).
Status: bench, two tapes, both recovered.
Volume table entry¶
Bytes 0–56 are the standard VTBL entry. The vendor-specific bit is clear, yet bytes 57–127 do not follow QIC-80-MC Rev N:
| Offset | Size | Field | Rev N has here | Notes |
|---|---|---|---|---|
| 57 | 1 | Multi-cartridge sequence | Same | 01 |
| 58 | 3 | Mark | Vendor extension data | MTN (4D 54 4E) |
| 92 | 4 | Directory section size | Same | |
| 96 | 4 | Data section size | Quadword at 96–103 | A doubleword: the label starts at 102 |
| 100 | 2 | Unknown | ||
| 102 | 16 | Source volume label | Label at 106 | Four bytes earlier than Rev N |
| 118 | 2 | Unknown | ||
| 120 | 1 | Compression | Compression at 124 | 81: compressed, QIC-123 code 1 (QIC-122) |
Read with Rev N's offsets, the same record claims a data section of billions of gigabytes. Read with these, one bench volume holds about 164 MB of data in about 98 MB of tape, a 1.7:1 ratio typical of QIC-122. That plausibility is what Tapewyrm's profile scoring checks.
Compression extents¶
The volumes use QIC-122 frames as in QIC-113 Rev G §9, with one difference:
| Field | QIC-113 Rev G | MTN |
|---|---|---|
| Uncompressed volume byte offset | 8 bytes | 4 bytes |
| Frame size | 2 bytes | 2 bytes |
With an 8-byte offset, the first frame size lands inside the offset field
and nearly every segment fails to decode. With 4 bytes, each segment's
offset equals the sum of the output of the segments before it. The profile
sets [extent] offset_bytes = 4.
The directory is stored as raw frames at the start of the volume, so the data section starts at an uncompressed offset equal to the volume table's directory section size.
Volume contents¶
The volumes are QIC-113 Basic DOS, directory first (directory-last bit clear). They follow the standard in places where Tapewyrm's first reader did not, and differ from it in two:
| Item | QIC-113 Rev G | MTN tapes |
|---|---|---|
| Fixed portion size byte | 10 (includes the extra-info byte) | 9: no extra-info byte |
| End of directory | Last-in-table attribute bit | Zero padding; the bit is not set |
| Directory order | Example is depth-first over levels | Depth-first over levels |
| Data entry size | Includes the data header | Includes the data header |
| Path entry | Directory the item is in | Directory the item is in |
| Empty directories | Header-only data entry, non-zero size | Same; no directory level |
The qic113-basic parser reads the size byte to place the name portion, so
9-byte and 10-byte entries both parse without a setting.
Tapes seen¶
| Cartridge | Header | Notes |
|---|---|---|
| QIC-80 307.5 ft (DC2120), factory preformatted | Format code 2 (fixed), 28 × 150 | One volume. Header bytes 78 and 144–145 non-zero, which Rev N calls unused. |
| QIC-80 | Not recorded here | One volume. Some files lost where their data headers sat in unreadable segments. |