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SubjectRe: mode data=journal in ext3. Is it safe to use?
Hans Reiser wrote:

> Timothy Miller wrote:
>
>> Doesn't Reiser4 do wear-leveling for flash?
>
>
> No, we don't. We do have wandering logs, so it would be feasible to
> code, but bitmap blocks and super blocks get written to the same
> locations repeatedly.
>
> Actually, most compact flash devices DO do wear leveling, from what I
> have heard.


The ones I've seen, only sort of. They'll allocate writes from
available erased pages to try to distribute their use, but if you
have a disk that's, say, 70% read-only data and 30% read-write then the
wear-levelling will only happen on that
30% of the disk. True wear levelling will actually scrub read-only or
rarely-written data, forcing it to get off its
duff so the flash cells they're sitting on can get some exercise, and
give the more worn cells a rest (that scrub
helps ECC fix soft errors from weak cells too). True wear-levelling is
really hard, and obviously requires
budgeting extra bandwidth and storage devices for safely shuffling
around data that the application has no
intention of moving (picture losing power in the middle of a scrub).
It's not worth it for the consumer CF
usage model of "take photos until the card is full, then copy them all
to the PC and wipe the card clean".

[Yes, I tend to see this from the inside-out: I'm actually an FPGA/ASIC
weenie not a kernel hacker. One of my current
projects is part of a controller chip for a solid-state storage system
with ${bignum} NAND flash chips. Alas, my specialty
is video and graphics, so I'm still coming up the learning curve on
storage systems].

ken



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