Poul-Henning Kamp wrote:
Aha, I understand now. CFA and SATA vendors have gone off in two
separate directions:
* PATA and SATA drives, for a few years now, have tended to rewrite one
cylinder at a time, which implies erasing the data on that cylinder.
Everybody denies this in the stongest possibly way whenever I ask them,
so far I have not seen this claim substantiated by any fact or person
who would be in a position to know.
Is this a case of blanket denial on the part of the disk manufacturers?
The feedback I've seen in FreeBSD forums regarding ATA write caching
tends to back up your original assertion.
I wonder if vendor neutral, reproducible, scientific research has been
conducted into this issue.
* NAND Flash devices should not have their sectors erased unless
absolutely necessary, to implement wear levelling.
Wrong, almost exactly the opposite in fact:
Flash devices using wear-levelling should have data erased as soon as
possible to give the wear-levelling the maximum amount of information
and available space to work with.
Ah, let me rephrase, I meant:
* NAND Flash embedded ATA controllers should not erase sectors
containing data unless absolutely necessary, to implement wear levelling.
* BIO_DELETE provides the necessary hint from the OS, by way of the ATA
CFA ERASE command, to tell the flash controller that the upper layer
consumer of the blocks has marked the data as being erased.
* The NAND flash ATA controller is *then* in a position to know how
best to implement that wear levelling as the OS has told it "I'm not
using these sectors any more".
Is this a correct summary?
[I came across an alternative to JFFS2 awhile back whose name escapes
me, which might work for the SDIO arm stuff imp was playing with.]
BMS
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