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Add fastlz compression support

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This commit is contained in:
Lucian Petrut
2026-09-07 13:10:10 +00:00
parent b7131ec0af
commit 56a1c36c80
13 changed files with 522 additions and 71 deletions
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docs/nfc_read.md
+29 -10
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@@ -39,21 +39,40 @@ LBA 8 on this disk was 4096 zero bytes on the wire, not a skip.
Little-endian, after the usual 16-byte AIO header
(`magic 0xA100DA7A`, type 7, size 44, monotonic `opId`):
| Offset | Type | VDDK `Read(start, n)` |
| ------ | -------- | ---------------------------------------------- |
| 0 | `uint64` | File handle from `OPEN_FILE` |
| 8 | `uint64` | `1` (`NFC_AIO_IO_READ`; write uses `0`) |
| 16 | `uint64` | Byte offset |
| 24 | `uint64` | Byte length |
| 32 | `uint32` | Byte length (same value) |
| 36 | `uint32` | Byte length (same value) |
| 40 | `uint32` | `0` (flags; uncompressed in this capture) |
| Offset | Type | VDDK `Read(start, n)` |
| ------ | -------- | ---------------------------------------------------------- |
| 0 | `uint64` | File handle from `OPEN_FILE` |
| 8 | `uint64` | Direction in low 32 bits; FastLZ type `2` in high 32 bits |
| 16 | `uint64` | Byte offset |
| 24 | `uint64` | Byte length |
| 32 | `uint32` | Byte length (same value) |
| 36 | `uint32` | Byte length, or compressed extra size when type is FastLZ |
| 40 | `uint32` | `0` |
An earlier guess that offset 36 was `NFC_DISK` (`2`) was wrong: a
1-sector VDDK read puts `512` in both `uint32` length fields. A Python
read that sent `(512, 2, 0)` still worked for one sector; the
replacement now matches VDDK.
`VIXDISKLIB_FLAG_OPEN_COMPRESSION_FASTLZ` does not change OPEN_FILE
flags. The IO opcode at offset 8 is a `uint64`: low 32 bits are still
`0`/`1` (write/read), high 32 bits are the NFC compression type
(`2` = FastLZ). OPEN still uses handshake `PlainText`.
| Open flag / wire | Request extra | Reply extra |
| ---------------------------------------- | ------------------------------------- | ------------------------------------------------ |
| No compression flag | Raw `length` bytes on write | Raw fragment at offset 32 |
| FASTLZ, data that shrinks | FastLZ bytes; offset 36 = packed size | Opcode type `2`; extra is FastLZ of offset 32 |
| FASTLZ, incompressible | Raw bytes; opcode type `0` | Opcode type `0`; extra is raw |
Reads with FASTLZ always *request* type `2`. The server may answer type
`2` or fall back to type `0`. Decompress into the uncompressed fragment
length at offset 32 and copy to the dest at offset 28.
64 KiB chunks use FastLZ level 2 (first byte has bit 5 set). Smaller
chunks use level 1. VDDK’s URL form is `FASTLZ-vpxa-nfc://…`; authd
`PROXY` is unchanged.
## Reply
Each fragment is: 16-byte AIO header (same `type` and `opId`) + 44-byte
@@ -117,7 +136,7 @@ fragments).
## What is still VDDK-only
- Compression flags on the last `uint32`
- zlib and skipz NBD compression flags
- `VixDiskLib_ReadAsync` (same IO messages, different client threading)
- `VixDiskLib_QueryAllocatedBlocks` / allocation bitmaps
- `VixDiskLib_GetInfo` capacity (not required to read a known range)