perf: allocate a chain's sectors for a whole write at once - #86
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A write now allocates all the sectors it needs first, writes their FAT entries together, and skips zeroing sectors it fills completely.
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Growing a chain cost a FAT entry write for the new sector, another to link it, and a zero fill, per sector. A write past the end of a chain now takes all the sectors it needs first (
Allocator::extend_chain_by), writes the FAT entries of each run of consecutive ids together, and skips the zero fill of sectors it fills completely; the unused tail of the last sector andset_lengrowth are still zeroed.Measured against master with #85 merged:
It also removes the zero fill that #87's direct writes pay up front, which had made the single 256 MiB write to disk slower than 0.14.0 (136 ms to 184 ms); with this change it takes 117 ms.