preerase: Implemented the gc side of preerase
Allocating pre-erased blocks gets quite complicated due to our
restricted flash model, but at least the actual pre-erasing is
relatively straightforward:
- We keep track of known preerased state in lfs3->gbmap.preeraser.
- If LFS3_GC_PREERASE is provided during gc work, we increment the
preeraser's known window by scanning the gbmap.
- Any BMFREE ranges we find, we erase a block at a time, and store the
resulting ecksum in a BMERASED range in the gbmap.
- We keep track of how many blocks we erased, and stop early if this
exceeds cfg.gc_preerase_count. This just lets users tune how many
blocks to preerase in case something (?) prevents preerased blocks
from being used.
Some notes:
- We don't really do anything with ranges in lfs3_alloc_preerase. In
theory we could bulk in erase to minimize the number of commits to the
gbmap, but we expect erase to dominate, so this probably isn't worth
it.
And if erase doesn't dominate, why would you bother pre-erasing
blocks?
- Preerasing isn't really a traversal operation, and is managed by a
sort of secondary state machine in lfs3_fs_gc_.
This also means lfs3_trv_read with LFS3_T_PREERASE does nothing, but I
guess that is ok? It's tempting to try to make lfs3_trv_read also
preerase, but it's unclear what block it should return -- it's
probably the wrong API.
- Introducing ecksums actually went quite a bit smoother than I
expected. Though it helps ecksums are the only optional payload, no
type punning or anything.
Ecksums do muddy the gbmap's design a bit, unfortunately. The main
issue being that we can only merge BMERASED ranges with equal ecksums.
This makes BMERASED ranges less compressable than the others, and may
be one reason to limit cfg.gc_preerase_count.
However:
1. This is where I think it's useful to emphasize that the gbmap's
responsibility is to track _free_ blocks, in-use blocks are
secondary.
When allocating, we're going to stop at the first BMFREE/BMERASED,
but may need to skip over an unbounded number of BMINUSE/BMBAD
blocks. So the compressability of BMFREE/BMERASED ranges should
have less of an impact on block allocation.
2. In practice, most flash uses consistent erase values, so the
resulting ecksums will probably be compressable. The exceptions are
noop-erases (SD/eMMC, RAM, NVRAM, etc), and encryption with block
address permutation?
Though noop-erases are a pretty big exception.
Code changes:
code stack ctx
before: 35116 2136 660
after: 35116 (+0.0%) 2136 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap+np before: 38040 2136 776
gbmap+np after: 38188 (+0.4%) 2144 (+0.4%) 776 (+0.0%)
code stack ctx
gbmap+yp before: 38040 2136 776
gbmap+yp after: 38608 (+1.5%) 2144 (+0.4%) 796 (+2.6%)
This commit is contained in:
Christopher Haster
+11
-5
@@ -84,11 +84,12 @@ F_CKDATACKSUMS = 0x01000000 # y- Check data checksums on reads
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F_MKCONSISTENT = 0x00000100 # y- Make the filesystem consistent
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F_LOOKAHEAD = 0x00000200 # y- Repopulate lookahead buffer
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F_PREERASE = 0x00000400 # y- Try to pre-erase free blocks
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F_COMPACT = 0x00000800 # y- Compact metadata logs
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F_CKMETA = 0x00001000 # y- Check metadata checksums
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F_CKDATA = 0x00002000 # y- Check metadata + data checksums
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F_CK = 0x00003000 # a- Alias for all check work
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F_GC = 0x00003b00 # a- Alias for all gc work
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F_GC = 0x00003f00 # a- Alias for all gc work
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# Filesystem mount flags
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M_MODE = 1 # -m Mount's access mode
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@@ -105,29 +106,32 @@ M_CKDATACKSUMS = 0x01000000 # y- Check data checksums on reads
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M_MKCONSISTENT = 0x00000100 # y- Make the filesystem consistent
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M_LOOKAHEAD = 0x00000200 # y- Repopulate lookahead buffer
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M_PREERASE = 0x00000400 # y- Try to pre-erase free blocks
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M_COMPACT = 0x00000800 # y- Compact metadata logs
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M_CKMETA = 0x00001000 # y- Check metadata checksums
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M_CKDATA = 0x00002000 # y- Check metadata + data checksums
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M_CK = 0x00003000 # a- Alias for all check work
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M_GC = 0x00003b00 # a- Alias for all gc work
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M_GC = 0x00003f00 # a- Alias for all gc work
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# File/filesystem check flags
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CK_MKCONSISTENT = 0x00000100 # -- Make the filesystem consistent
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CK_LOOKAHEAD = 0x00000200 # -- Repopulate lookahead buffer
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CK_PREERASE = 0x00000400 # -- Try to pre-erase free blocks
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CK_COMPACT = 0x00000800 # -- Compact metadata logs
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CK_CKMETA = 0x00001000 # -- Check metadata checksums
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CK_CKDATA = 0x00002000 # -- Check metadata + data checksums
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CK_CK = 0x00003000 # a- Alias for all check work
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CK_GC = 0x00003b00 # a- Alias for all gc work
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CK_GC = 0x00003f00 # a- Alias for all gc work
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# GC flags
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GC_MKCONSISTENT = 0x00000100 # -- Make the filesystem consistent
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GC_LOOKAHEAD = 0x00000200 # -- Repopulate lookahead buffer
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GC_PREERASE = 0x00000400 # -- Try to pre-erase free blocks
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GC_COMPACT = 0x00000800 # -- Compact metadata logs
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GC_CKMETA = 0x00001000 # -- Check metadata checksums
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GC_CKDATA = 0x00002000 # -- Check metadata + data checksums
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GC_CK = 0x00003000 # a- Alias for all check work
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GC_GC = 0x00003b00 # a- Alias for all gc work
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GC_GC = 0x00003f00 # a- Alias for all gc work
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# Filesystem info flags
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I_RDONLY = 0x00000001 # -- Mounted read only
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@@ -144,6 +148,7 @@ I_CKDATACKSUMS = 0x01000000 # -- Mounted with LFS3_M_CKDATACKSUMS
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I_MKCONSISTENT = 0x00000100 # -- Filesystem needs mkconsistent to write
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I_LOOKAHEAD = 0x00000200 # -- Lookahead buffer is not full
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I_PREERASE = 0x00000400 # -- Blocks can be pre-erased
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I_COMPACT = 0x00000800 # -- Filesystem may have uncompacted metadata
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I_CKMETA = 0x00001000 # -- Metadata checksums not checked recently
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I_CKDATA = 0x00002000 # -- Data checksums not checked recently
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@@ -156,11 +161,12 @@ T_MTREEONLY = 0x00000002 # -- Only traverse the mtree
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T_EXCL = 0x00000008 # -- Error if filesystem modified
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T_MKCONSISTENT = 0x00000100 # -- Make the filesystem consistent
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T_LOOKAHEAD = 0x00000200 # -- Repopulate lookahead buffer
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T_PREERASE = 0x00000400 # -- Try to pre-erase free blocks
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T_COMPACT = 0x00000800 # -- Compact metadata logs
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T_CKMETA = 0x00001000 # -- Check metadata checksums
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T_CKDATA = 0x00002000 # -- Check metadata + data checksums
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T_CK = 0x00003000 # a- Alias for all check work
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T_GC = 0x00003b00 # a- Alias for all gc work
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T_GC = 0x00003f00 # a- Alias for all gc work
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t_TYPE = 0xf0000000 # im The traversal's type
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t_REG = 0x10000000 # i^ Type = regular-file
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