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CompactFlash for storage
[ Message cross-posted to comp.arch.embedded and comp.arch.storage ]
Hello everyone, I have an x86-based system running Linux, and I'm considering using a CompactFlash card for non-volatile storage, instead of a typical hard disk drive. I was wondering if I could just use the same IDE driver I've been using for hard disk drives. As far as I understand, if I use a CF-to-IDE adapter, and plug that adapter into the IDE connector, then there is no problem. However the motherboard provides a "CF socket", so I was thought I might not need the CF-to-IDE adapter. If I stick a CF card in the CF socket, will the regular IDE driver work, or do I need one of the MTD drivers? Note: the datasheet states "CompactFlash Socket on IDE 0 (Primary/Master)" which seems to indicate the CF socket is just an embedded CF-to-IDE adapter, and everything should work, right? Another question: I've been using so-called DOMs (Disk On Module) which often include some form of wear-leveling to even the wear of different memory cells. Do typical CF cards include this feature? How much electronics (other than plain storage) is there in a CF card? Are features such as DMA support implemented in the card or are they supported at a different level? (In the IDE controller? Elsewhere?) Regards. |
#2
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CompactFlash for storage
On Sep 4, 5:51 am, Spoon root@localhost wrote:
However the motherboard provides a "CF socket", Without exception that I have seen, on-board CF sockets on x86 systems are already wired in True-IDE mode. embedded CF-to-IDE adapter, and everything should work, right? The "adapter" is a single strapped pin that puts the device in True- IDE mode. Otherwise it's just a connector. Another question: I've been using so-called DOMs (Disk On Module) which often include some form of wear-leveling to even the wear of different All CF cards include wear leveling. However the strategies you use when running off flash are very different from what you'd use running off rotating magnetic media. Run some quick back-of-an-envelope calculations and you'll see that even a seemingly-infrequent regular write operation can VERY quickly eat the erase lifetime of typical NAND flash chips. Also: There are bonus gotchas when using CF media. * Writes are slower than reads, and much more power-hungry. * It used to be true - and I believe from recent experiment is still true - that a power failure during write operations can leave the card unusable. Not merely corrupt filesystem, but actually a useless card. * In True-IDE mode, you cannot hot-swap cards. * {etc...} How much electronics (other than plain storage) is there in a CF card? A typical CF card is more or less three chips: voltage regulator, NAND flash [might be more than one physical chip], and microcontroller. The performance of the CF card is heavily dependent on the capabilities of the micro. Some companies even used to advertise "USES FAST H8 MICROCONTROLLER!". supported at a different level? (In the IDE controller? Elsewhere?) There is no "IDE controller" per se in a CF card, there's only the micro. |
#3
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CompactFlash for storage
In article 46DD2AA2.4040205@localhost, Spoon root@localhost wrote:
Another question: I've been using so-called DOMs (Disk On Module) which often include some form of wear-leveling to even the wear of different memory cells. Do typical CF cards include this feature? Typical IDE DOMs and CF cards are the exact same item in a slightly different shape, particularly if you go with the so-called "industrial" ones from the same manufacturer. But one gotcha with motherboard CF sockets (this is what I warned about before) is that some board makers, for reasons that are unclear to me, do not actually run the signal lines needed for DMA to the CF socket, so, on the same board, a DOM with the exact same flash, microcontroller, and firmware _may_ be faster: you should test for yourself. -- Thor Lancelot Simon "The inconsistency is startling, though admittedly, if consistency is to be abandoned or transcended, there is no problem." - Noam Chomsky |
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