Normally, count=n means read(2) will be called n times on the input to dd. If the read() returns short, as may happen when reading from a pipe, fewer bytes will be copied from the input. With conv=sync the buffer is padded with zeros to fill the rest of the block.
iflag=fullblock causes dd to continue reading until the block is full, so that count=n means n full blocks are copied. This flag is compatible with illumos and GNU dd and is used in the ZFS test suite.
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