tar fs copy

[Seth H Zirin]

> > How big is the pipe?  I read somewhere pipes use the root file system,
> > which on our computer has about 1100 free blocks.  Yet, I have used
> > cphier to transfer, for example, the entire netnews source, including
> > rn, from one drive to another.  All I created was the highest level
> > directory.  Cphier made all the rest.  Will it sometimes work and sometimes
> > fail on a transfer of this size?
> 
> Pipes use an inode on the root file system. They are limited to 5120 bytes
> on System V, and 4096 on some other systems, by code in the kernel.

Not entirely true, pipes do use an inode, but its from the "pipedev" not
the "rootdev" in SIII and SV.  The "pipedev" is however usually the same
as the "rootdev", although it doesn't have to be.  The data in pipes is
stored in blocks of the buffer cache.  When the buffer cache fills and a
buffer is needed for perhaps another process, the data from a pipe could
find itself physically on the disk (especially with named pipes).  I have
seen instances (SIII) where the system crashed or was rebooted when there
was data in opened named pipes, and the data remained accessable after
reboot.  If both ends of the named pipe are closed before reboot, the data
is flushed.
-- 
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Name:	Seth H Zirin
UUCP:	{decvax, ihnp4}!philabs!sbcs!megad!seth

Keeper of the News for megad