On Mon, 29 Nov 2004, Alan Stern wrote: > On Mon, 29 Nov 2004, Sara Fonseca wrote: > > > how is the address mapping usually done in a block device? > > Normally the physical and logical block numbers are the same. > > > (since all > > SCSI commands see are logical blocks, the physical address that > > actually stores the logical block information shouldnt be always the > > same, should be dynamic (so you don't keep writing some physical > > blocks more frequently than others)). > > That is wrong. There are only a few reasons why physical block numbers > might be different from logical block numbers. The two most common are: > > So that the drive can store private data, not accessible by > logical block commands. > > So that if a medium error develops in some physical block, the > corresponding logical block can be remapped to a different > address. > > Yes, it is true that this means some physical blocks are written more > frequently than others. Disk drives seem to work okay, regardless.
For some flash memory media, a third reason is to reduce the frequency of writing the same physical block, I hear. In some memory cards, the internal controller dynamically maps logical blocks to physical ones and stores the mapping information in private internal memory. Anyway, at the SCSI level you only can only address the logical blocks. That is unless you know the vendor-specific commands, if any, to read physical blocks. :-) Best regards, jim. Jim Blackson Y-E Data, Inc. maker of 7-in-1 Digital Media memory card Reader/Writer http://www.yedata.com/ ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://productguide.itmanagersjournal.com/ _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
