When does `write' return 181

Andrew Xue Sorry. That was a DEC thing on their hard drives in the very late 1960s, at copy-on-write.

Their disks had 512-byte sectors and UNIX at the time normally read in one sector at a time, and if it detected sequential reading, it might read the next block as well.

But the hardware controller for the disk drives allowed you to read any multiple of 512 bytes you wanted. I do not remember the numbers, so I will make them up here, but you will get the idea. Imagine there are 19 tracks per cylinder and 20 sectors per track. If you started at block zero and read 512 * 20 * 19 bytes, you would read the whole cylinder. But if you read 512 * 20 * 29 bytes, the controller would accept that, read the whole cylinder and do an automatic seek with no intervention from the OS and read about half the next cylinder as well. You could go as much as you wanted, up to almost 65536 bytes (which would use up all the data space in any one process).

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