62 lines
2.4 KiB
Plaintext
62 lines
2.4 KiB
Plaintext
This is a device mapper block compression driver.
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Note:
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- Sectors are always 512 bytes (kernel constant).
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- Physical blocks are variable 512b .. 4kb.
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- Logical blocks are variable 4kb .. 4mb.
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- All integers are little endian.
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Block device layout:
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- pblk[0] header (512 bytes)
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/* Offset 0: magic and version */
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- byte[4] magic
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- u16 version_major
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- u16 version_minor
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/* Offset 8: parameters */
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- u16 flags
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- u8 compression
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hi nybble: algorithm (1=lz4, 2=zlib, ...) [1]
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lo nybble: level (1..9) [1]
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- u8 pblk_shift (0..3) [3 = 4kb]
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- u8 lblk_shift (1..10) [4 = 64kb (*)]
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- u8 lba_elem_pblk_bytes (2, 4, 6)
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- u8 pbat_shift [0]
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- u8 pad
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- u32 nr_zones
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- u32 lblk_per_zone
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- byte[40] reserved
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/* Offset 64: stats */
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- u64 pblk_used
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- u64 lblk_used
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- byte[48] reserved
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/* Offset 128: reserved */
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- byte[384] reserved
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- pblk[] zone ... vector of zone
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(*) The maximum lblk_shift may be (and usually is) less than 10 because
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lblk_alloc size must be <= pblk size. When pblk_shift=1 (1024) and
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lblk_alloc_16_32, the maximum lblk_shift is 8 (256kb).
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Device size limits:
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- Min: header plus one zone (header, pbat, lbat, data)
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Arbitrarily limit to 1mb (?)
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- Max: 2^48 pblk = 1eb
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There are six different lblk_alloc sizes:
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Struct name Size lblk_size device_pblk_size
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=========== ==== ========= ================
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lblk_alloc_16_16 2+2*8 <= 64kb <= 64k
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lblk_alloc_16_32 2+4*8 <= 64kb <= 4g
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lblk_alloc_16_48 2+6*8 <= 64kb <= 256t
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lblk_alloc_32_16 4+2*8 > 64kb <= 64k
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lblk_alloc_32_32 4+4*8 > 64kb <= 4g
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lblk_alloc_32_48 4+6*8 > 64kb <= 256t
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Zone layout:
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- byte[pblk_size*pbat_len] Physical block allocation table (pbat)
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- lblk_alloc_x_y[] Logical block allocation table (lbat)
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Note: padded to pblk size
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- data[] Data
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One pblk per bit in pbat
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