Samsung's F2FS filesystem
Posted Oct 9, 2012 15:45 UTC (Tue) by dwmw2
In reply to: Samsung's F2FS filesystem
Parent article: Samsung's F2FS filesystem
It's an attempt to work around the fact that the FTL found in devices like eMMC is generally really low quality.
So while in theory you can treat it as "just a disk", in practice that's almost never the case. Even in theory that fantasy doesn't hold true once you start paying attention — even a decent high-end SSD needs layering violations like TRIM in order to maintain efficiency and stop it from garbage collecting (and thus suffering write amplification) on data it doesn't even need any more.
So Samsung have come up with a "disk" file system which is optimised to work around the failings of the eMMC devices that they are using. Which is all very well for Samsung, who make their own MMC devices too. But the problem for everyone else is that you don't actually know about the internals of one of these devices. It's just a black box to you. You could do extensive testing and find one of the rare ones that does actually survive extended powerfail testing, and that works efficiently with this type of software workaround — but manufacturers have a history of changing the internals dramatically without even changing the model number, so the next batch you order could be completely different. Instead of aligning your the allocation units of your "disk" file system to carefully line up with NAND eraseblock, due to your internal knowledge of the the way the internal FTL happens to lay stuff out, you could suddenly find that your "disk" file system is laying stuff out precisely wrong for the lower layer instead.
Any optimisation you attempt at this level is a layering violation and is doomed from the beginning unless you can control the internals of the MMC device too. Which you can't. Although Samsung can. So that's nice for them.
Really, people who are serious about embedded Linux should be driving the NAND directly rather than accepting the MMC "pretend to be spinning rust" approach, with all the disadvantages and unreliability that it brings.
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