Date: Mon, 17 Dec 2007 17:09:31 +0100 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: Bruce M Simpson <bms@incunabulum.net> Cc: cvs-src@FreeBSD.org, Poul-Henning Kamp <phk@phk.freebsd.dk>, src-committers@FreeBSD.org, "Bruce M. Simpson" <bms@FreeBSD.org>, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sbin/atacontrol atacontrol.c Message-ID: <86hcihi9qc.fsf@ds4.des.no> In-Reply-To: <4766951B.8090504@incunabulum.net> (Bruce M. Simpson's message of "Mon\, 17 Dec 2007 15\:26\:19 %2B0000") References: <11419.1197903331@critter.freebsd.dk> <4766951B.8090504@incunabulum.net>
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Bruce M Simpson <bms@incunabulum.net> writes: > The feedback I've seen in FreeBSD forums regarding ATA write caching > tends to back up your original assertion. It wouldn't be the first case of uninformed people reinforcing eachother's belief in incorrect information. > I wonder if vendor neutral, reproducible, scientific research has been > conducted into this issue. Scientific research? Since when does reverse engineering qualify as scientific research? > * NAND Flash embedded ATA controllers should not erase sectors > containing data unless absolutely necessary, to implement wear > levelling. No. Blocks which go from used to unused should be immediately erased. Otherwise, the device fills up with unused, unerased blocks and you end up having to erase a block every time you write to the device. Remember that cells in NAND flash can only be changed in one direction (one to zero for single-level flash) without being erased. As a result, there are very few cases (allocation bitmaps being one of them) where you can write to the same block of NAND flash multiple times without erasing it in between. > [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.] Specialized file systems such as JFFS and YAFFS are not very useful for flash media with built-in wear levelling, such as USB sticks or CF / SD cards. They are mostly useful for embedded devices with on-board NAND flash. DES --=20 Dag-Erling Sm=C3=B8rgrav - des@des.no
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