From owner-freebsd-hackers@FreeBSD.ORG  Sun Oct 25 19:40:29 2009
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Date: Sun, 25 Oct 2009 13:34:37 -0600 (MDT)
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To: usb@FreeBSD.org
From: "M. Warner Losh" <imp@bsdimp.com>
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Cc: hackers@FreeBSD.org
Subject: Help troubleshooting...
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I have a usb stick (8GB) on it.  This stick has about 5GB of junk on
it at this point.

I tried to do 'cat * > /dev/null' recently, to measure how fast it
goes.  It got about 1GB into the drive and then I got device missing
messages.

Here's the dmesg messages:

# Plug it in
da0 at umass-sim0 bus 0 scbus2 target 0 lun 0
da0: <HP v125w PMAP> Removable Direct Access SCSI-0 device 
da0: 40.000MB/s transfers
da0: 7660MB (15687680 512 byte sectors: 255H 63S/T 976C)
# mount it, etc
# run the cat command
Device da0s1 went missing before all of the data could be written to it; expect data loss.
# get error messages
# Remove the drive
ugen2.2: <HP> at usbus2 (disconnected)
umass0: at uhub2, port 1, addr 2 (disconnected)
(da0:umass-sim0:0:0:0): lost device
(da0:umass-sim0:0:0:0): removing device entry

So devfs thinks the device went missing:

static int
devfs_fsync(struct vop_fsync_args *ap)
{
...
	if (!vn_isdisk(ap->a_vp, &error)) {
		bo = &ap->a_vp->v_bufobj;
		de = ap->a_vp->v_data;
		if (error == ENXIO && bo->bo_dirty.bv_cnt > 0) {
			printf("Device %s went missing before all of the data "
			    "could be written to it; expect data loss.\n",
			    de->de_dirent->d_name);
...

So it thinks that it isn't a disk.  vn_isdisk is return ENXIO because
either vp->v_rdev == NULL or the v_rdev->si_devsw == NULL.

So how the heck can that happen without other warnings?  It appears
the only place it is set like this is in devfs_reclaim.  But I'm
having trouble tracking down where *THAT* is called.

This is with the following system:

FreeBSD lighthouse 9.0-CURRENT FreeBSD 9.0-CURRENT #41 r185338:198411M: Fri Oct 23 10:08:48 MDT 2009     imp@lighthouse:/cache/svn/head/sys/amd64/compile/LIGHTHOUSE  amd64

Warner