03-23-2006
character driver (rdsk)
The driver has read and write entry points. The read and write system calls pass this off to the driver. The driver decides if the operation succeeds or not. If it fails, the driver can set any errno value it wants. There are standards, but some driver writers do not follow them. There are rules to follow and they can vary from driver to driver. Typically you must start a read or write on a disk sector boundary. And you must write full sectors. If you try to write a partial sector, and the driver allows it, you will probably garble the rest of the sector. Data moves directly to/from the disk to/from your program. There is no synchronization with data in the buffer cache.
block driver (dsk)
The driver has no read or write entry points. You cannot talk to the driver in a direct manner. The kernel must prepare a buffer header to pass to the driver's strategy entry point. read() and write() should strongly follow the man page. No sector boundary rules. You can write a single byte and it should work. Data must flow to/from the buffer cache and then to/from your program; and thus is in sync with any recent changes.
(Note that the "in sync" implies operations to the same device. If you mount a slice of a disk, then dd the whole disk via another special file, it won't be in sync.)
I would always use rdsk to avoid copying the data around in core. Your 9 hours vs 15 minutes is very extreme. You don't give any particulars of your situation. But it does not take 8 hours and 45 minutes to do the in-core copying of data that can be read in 15 minutes. I don't enough info to guess what happened.
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