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Improving ext4: bigalloc, inline data, and metadata checksums

Improving ext4: bigalloc, inline data, and metadata checksums

Posted Dec 4, 2011 19:38 UTC (Sun) by dlang (subscriber, #313)
In reply to: Improving ext4: bigalloc, inline data, and metadata checksums by tytso
Parent article: Improving ext4: bigalloc, inline data, and metadata checksums

if you have 4K of data that is part of an eraseblock that you modify, that eraseblock now no longer contains valid info, so since it can't overwrite the 4K it will need to write to a new eraseblock.

yes, in theory it could mark that 4k of data as being obsolete and only write new data to a new eraseblock, but that would lead to fragmentation where the disk could have 256 1M chunks, each with 4K of obsolete data in them, and to regain any space it would then need to re-write 255M of data.

given the performance impact of stalling for this long on a write (not the mention the problems you would run into if you didn't have that many blank eraseblocks available), I would assume that if you re-write a 4k chunk, when it writes that data it will re-write the rest of the eraseblock as well so that it can free up the old eraseblock

the flash translation layer lets it mix the logical blocks in the eraseblocks, and the drives probably do something in between the two extremes I listed above (so they probably track a few holes, but not too many)


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