Anne Wilson posted on Tue, 29 May 2012 15:56:00 +0100 as excerpted:
> I would have thought that a hot-plug device would look, to the system,
> like a removable drive.
AFAIK, this is a very commonly held misconception, as the "removable"
attribute refers to the media itself, not the full device.
Think of it this way:
* Optical media comes out of the device (the CD/DVD reader), so it's
removable.
* Various magnetic-media zip drives, ls120 (IIRC that's what they were
called, 120 MB removable media, like zip, was supposed to replace the
floppy) drives, sparq drives (I had a sparq, IIRC 1 GB capacity, at one
point) are removable, since the media physically comes out of the drive,
which stays behind.
* Conventional 2.88 MB and smaller floppies would be removable, except
they predate the standard and work on a different bus, the floppy bus.
* USB thumbdrives aren't removable (tho sometimes their firmware lies and
says they are), despite being hotpluggable, because the media doesn't
come out of the "drive", the drive itself detaches.
* Similarly, the common "portable USB drives" aren't removable, since the
drive itself detaches, the media staying in it.
Meanwhile, these days pretty much everything (well, we're talking block
devices here, but the same idea applies to most devices in general, these
days, even sound and graphics cards are USB-attachable these days) is
hotplug-capable in one form or another. Obviously anything USB-attached
is, but so are ESATA devices. Actually, most internal SATA devices are
electronically hotpluggable as well, even if they're screwed into the
case. I think it's actually part of the SATA standard so a SATA device
that's not hotpluggable isn't standard-compliant, but I could be wrong.
Certainly SCSI devices are hotpluggable and I'm almost certain it's part
of that standard. IEEE-1394 (aka firewire) is of course hotpluggable as
well.
The kernel actually deals with SATA bus malfunctions (and I think even
legacy PATA, these days, at least if the libata interface is used, the
legacy ide interface may not, however) by resetting the device, such that
it's redetected and reappears in the system. If the device was mounted
when it was removed/reset and it reappears soon enough, it's actually
often possible for the kernel to resume an I/O transaction, even a write
given a journaled filesystem like ext3/4/reiserfs/xfs/jfs or a "dancing
tree" filesystem like reiser4/btrfs, without damage and without the
userspace application even necessarily knowing (tho udev/udisks of course
knows).
Given that a modern kernel must be and generally is prepared for devices
to appear/disappear at any time, there really isn't that much (if any)
distinction between hotplugged and not-hotplugged, any more.
--
Duncan - List replies preferred. No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master." Richard Stallman
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