It probably matters which OS.  I would advise running ktrace/ktruss to
see which syscalls are made.  Coda has the notion of opening a file
for write, writes, and then a store.  Offhand, I don't know why
suspend would be an issues.  A process may be in the kernel waiting on
the venus pioctl for venus to respond to an operation, and that may
fail to be interrupted.

Also, I don't know if cp behaves well if some of the read/write
syscalls return EINTR, or if it just gives up.  It is unclear to me
how and to what extent a good program is supposed to cope with this
and retry.

On BSD, grok sys/coda/*.c, the VFS rules, and signal delivery.  After
several weeks of just reading code, you will be enlightened and able
to fix any issues that are present :-)

-- 
        Greg Troxel <gdt_at_ir.bbn.com>