Re: usefullness of a read-only block UBI interface ?
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On Tue, May 24, 2011 at 03:53:03PM +0100, David Wagner wrote: > Hello linux-mtd, -embedded and -fsdevel, > > There are a lot of actively developed block filesystems out there, more > than flash filesystems. Read-only block FS can run with great perfs on > flash supports with the mtdblock interface (eg. SquashFS) but since it > doesn't handle bad blocks, read will fail when you hit one. > > That's why we are considering the pros and cons of having a block > interface on top of UBI: UBI takes care of bad blocks and filesystems > above it don't have to worry about them. > > An option could be to implement bad block handling in mtdblock but > then, there wouldn't be any wear-leveling. Hello David, Handling bad blocks is one thing, but it would not be enough. I assume you want to use a nand device (bad blocks ?). Reading nand pages over and over will result in bitflips (so-called "read disturbs"). Those bitflips are corrected by ecc in mtd, but they must also be taken care of at a higher level, by (atomically) moving faulty data to another block and erasing the original block (which is enough to clear read disturbs). UBI does that in its block scrubbing operation. > In case of read-only filesystems, wear-leveling is not so important but > when read-only and read-write filesystems coexist, static wear-leveling > is important. And I understand that UBI implements static > wear-leveling. So it would make sense to have a block read-only > filesystem on top of UBI along with a ubifs read-write filesystem. Yes, especially if your read-only filesystem is very large; you need to spread wear-levelling across the entire device in order to maximize its lifetime. Regards, Ivan -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html