Re: Why doesn't 13_4 img (1.3G) write to a 4 G memstick?

Gary Aitken wrote:
> $ xz --decompress < /hd2/Downloads/FreeBSD/13_4/FreeBSD-13.4-RELEASE-amd64-memstick.img.xz | dd of=/dev/da0 bs=1M conv=sync

I don't like use of conv=sync coupled with dd reading from a pipe
especially when the source is a slow decompress utility.  Not what you
want there.

     conv=value[,value ...]
              Where value is one of the symbols from the following list.

              sync     Pad every input block to the input buffer size.  Spaces
                       are used for pad bytes if a block oriented conversion
                       value is specified, otherwise NUL bytes are used.

You really, really, really do not want to be padding short read blocks
in this situation.  If xz cannot keep up then dd will be padding a lot
of blocks and "creating" data.  Since you reported that it overflowed
your device that seems most likely to have happened here.  That will
never happen when reading from a file but might happen when reading
from a pipe with a slow source writing to it with a different block
size.

I think what you actually wanted was this.

    xz --decompress < /hd2/Downloads/FreeBSD/13_4/FreeBSD-13.4-RELEASE-amd64-memstick.img.xz | dd of=/dev/da0 bs=1M iflag=fullblock oflag=sync,direct

     iflag=value[,value ...]
              Where value is one of the symbols from the following list.

              fullblock  Reading from the input file may not obtain a full
                         block.  When a read returns short, continue reading
                         to fill the block.  Without this flag, count limits
                         the number of times read(2) is called on the input
                         rather than the number of blocks copied in full.  May
                         not be combined with conv=sync.

     oflag=value[,value ...]
              Where value is one of the symbols from the following list.

              sync    Set the O_SYNC flag on the output file to make writes
                      synchronous.  This is synonymous with the fsync value.

              direct  Set the O_DIRECT flag on the output file to make writes
                      bypass any local caching.

Bob