when we get eof, stop the loop immidiately and do not
rely on the read to eventually return an error.
when convM2S() fails to decode the message, error out
and stop the loop. there is no point in continuing.
data on the disk is layed out sequentially and directory information
is at the end of the disk. we want to keep data and metadata separated
so that reading large sequential files will not evict the directory
information from the cache causing long seeks.
for that, we tag the clusters (an 8th for metadata, and the rest
for data) and getbuf() will only evict clusters of the same tag.
doing tests taring up 9front.iso shows the following:
lowering the cluster size back to 128k avoids over half the
reads. 837888 sectors read for 512k vs. 347712 sectors with
128k cluster size.
we have to run the 9p service process in its own namespace
otherwise the attach filename might point onto the served
filesystem causing it to deadlock. this happens especially
if 9660srv is used as root filesystem. (cdboot)
