Improving ext4: bigalloc, inline data, and metadata checksums

Posted Dec 4, 2011 17:39 UTC (Sun) by tytso (subscriber, #9993)
In reply to: Improving ext4: bigalloc, inline data, and metadata checksums by dlang
Parent article: Improving ext4: bigalloc, inline data, and metadata checksums

Flash erase blocks are around a megabyte these days. So all modern SSD's use a Flash Translation Layer (FTL) that allows writes smaller to than an erase block to get written together in a single erase block. So it's simply not true that if you do a small random write of 16k, that the SSD will need to copy all of the other disk blocks that form up the erase block.

This might be the case for cheap MMC or SD cards that are designed for use in digital cameras, but an SSD which is meant for use in a computer will have a much more sophisticated FTL than that.

Improving ext4: bigalloc, inline data, and metadata checksums

Posted Dec 4, 2011 19:38 UTC (Sun) by dlang (guest, #313) [Link]

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)