df1394eaa3
SCFF, the SchOS Compressed File Format (include/fs/scff.h), is a block image cut into 128 KB chunks, each its own zstd frame, behind an index. tools/mkscff writes it from fs-ram.img on every core (-19, 11 s) and checks each chunk decompresses to itself. The kernel carries the .scff as its ramdisk and unpacks a chunk the first time a block in it is read; zero blocks share one page. The kernel and the ISO are the compressed size, and the root costs memory only for what is touched. See docs/architecture/scff.md. zstd 1.5.7 comes from ports/zstd like uACPI: the decoder is compiled into the kernel at -O2, with a static context and a 64 KB stack of its own, and mkscff builds the whole library from the same release. world now has xorg-server, the fbdev and evdev drivers, xinit, Window Maker, xterm and DejaVu, and /root/.xinitrc runs wmaker. That root is 358 MB, 102 MB as SCFF. The ISO no longer carries the root a third time as an ISO 9660 tree, which nothing read. schos.iso: 1389 MB -> 318 MB. El Torito keeps its own copy of the EFI image, the layout real machines have booted. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
54 lines
2 KiB
C
54 lines
2 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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// SCFF, the SchOS Compressed File Format: a block image cut into chunks, each
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// compressed on its own, with an index in front.
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//
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// It is what the live root is shipped as. The kernel carries the compressed
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// image and decompresses a chunk the first time any block in it is read, so
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// the kernel and the ISO are the compressed size, and the memory the root
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// takes is what has been touched, not the whole of it. See kernel/dev/ramdisk.c
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// for the reader and tools/mkscff.c for the writer; this header is the one
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// definition of the layout both of them use.
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//
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// offset 0 struct scff_header
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// index_off uint64_t off[nchunks + 1], from the start of the file
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// off[i]..off[i+1] chunk i
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//
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// A chunk is chunk_bytes of the image (the last one may be shorter). Its
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// stored length says how it is kept:
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//
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// 0 every byte is zero, and nothing is stored
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// its full length stored as it is: it did not compress
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// anything else one zstd frame
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//
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// Little-endian throughout, which is every machine SchOS runs on.
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#ifndef SCHOS_FS_SCFF_H
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#define SCHOS_FS_SCFF_H
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#include <stdint.h>
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#define SCFF_MAGIC "SCFF"
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#define SCFF_VERSION 1
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#define SCFF_CODEC_ZSTD 1
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// The kernel decompresses one chunk at a time into a buffer this big, so it
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// is the largest chunk a reader accepts. 128 KB is squashfs's default and
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// the point past which a bigger chunk buys little ratio for a lot of
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// decompression on every miss.
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#define SCFF_CHUNK_MAX (1u << 20)
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struct scff_header {
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char magic[4]; // "SCFF"
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uint16_t version; // SCFF_VERSION
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uint16_t codec; // SCFF_CODEC_ZSTD
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uint32_t chunk_bytes; // a power of two, a multiple of 4096, <= SCFF_CHUNK_MAX
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uint32_t nchunks;
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uint64_t image_bytes; // the uncompressed image
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uint64_t index_off;
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uint64_t stored_bytes; // the whole file, header and index included
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uint8_t reserved[24];
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};
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_Static_assert(sizeof(struct scff_header) == 64, "scff_header is 64 bytes");
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#endif
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