Re: [9fans] fossil shutdown

[Russ Cox <rsc@swtch.com>]

> > echo dma on >/dev/sdXX/ctl
> > echo rwm on >/dev/sdXX/ctl

> Inspired, I tried this, and I got an improvement (decrease)
> in time taken to copy files to a new filename of at least
> an order of magnitude.

> I was surprised to find the letters "rwm on" right at the
> beginning of the disk.  I saved the output of

> I'm pretty sure that I did *not* type
>
> echo rwm on > /dev/sdC0/data     /* WRONG */

I'm pretty sure that you did type

    echo dma on >/dev/sdC0/ctl
    echo rwm on >/dev/sdC0/data

explaining both the performance improvement
and the subsequently broken mbr.

The disk fragment you posted on the ftp server
begins with a standard NetBSD mbr except that
the first 7 bytes are "rwm on\n".  This is perfectly
explained by a typo and very hard to explain with
a software or hardware error.

Once I set up a pseudo-worm file server on a few
SCSI disks, spent a few hours populating it, and then
rebooted and the kernel couldn't find the file system.
On inspection, I found postscript code in what should
have been the file server's superblock.  That felt like a
hardware error (shouldn't use discarded machines;
people discard them for a reason!).  This doesn't.

Russ