SSD write leveling? [LWN.net]

SSD write leveling?

Posted Jul 4, 2008 10:02 UTC (Fri) by NRArnot (subscriber, #3033)
In reply to: SSD write leveling? by wtogami
Parent article: Acer's Linpus Linux Lite (Fedora) ultra portable laptop piles the pressure on Microsoft (FSM)

Isn't it also the case that a flash block (almost) always fails during write? So if the SSD
can't handle it with bad block relocation, the filesystem will  mark the block as bad and put
one's data elsewhere. Unless it's a really dumb filesystem that hammers on the one block that
can't be relocated (like FAT floppy disks did!)

It's far worse if a block is successfully written, and then goes bad later, as often happens
to hard disks when they start degrading mechanically.

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SSD write leveling?

Posted Jul 4, 2008 14:19 UTC (Fri) by Cato (subscriber, #7643) [Link]

It's worse than just losing a single block.  A flash erase block can include many filesystem
blocks as it's quite large - anything from 16 to 512 KB according to
http://en.wikipedia.org/wiki/Flash_memory#NAND_memories.  And in some devices with poor FTLs,
interrupting a single write can cause the whole device to fail - it's important that when the
device comes back up, the FTL checks for partially written erase blocks and marks them as bad.

Unfortunately FTLs appear to be mostly undocumented so you just have to rely on going for mid
to high end flash products from a reputable vendor, assuming you care about reliability.
Embedded flash products (e.g. CF drives) seem to have good reliability specs and may be a good
option, at some extra cost.