SATA (hot) swapping for backup

[Daniel Pittman]

David Abrahams <dave at boost-consulting.com> writes:
> Daniel Pittman <daniel at rimspace.net> writes:
>> David Abrahams <dave at boost-consulting.com> writes:
>>
>>> I recently bought a pair of SATA drives on which to do weekly backups
>>> of my server.  I also got a cool drive tray that allows me to pop the
>>> disks in and out of the machine easily:
>>> http://www.cooldrives.com/sata-serial-ata-mobile-rack-enclosure-lcd.html
>>> You can remove the drive from the machine by turning a key -- which
>>> powers it down -- and pulling on the handle.
>>>
>>> My problems: 
>>>
>>> 1. If I boot the server with no drive in the bay or with the drive
>>>    power off, it is never detected, even after powering it on.  Is
>>>    there a way to get that to work?

[...]

>> Sadly, none of that seems completely clear about what kernel release is
>> required for the various hot-plug and warm-plug options to work.  You
>> may be forced to resort to asking the linux-kernel mailing list if
>> someone else here can't chime in.
>
> Also, none of that is clear about what, exactly, constitutes "support
> for hot-swap" (which is connected with my next question).

Well, one key point is your issue above: while the hardware (probably)
issues an interrupt when the new hardware is connected, Linux ignores
that rather than detecting the device.

That would be one part of hot-swap - the hot-add case.

>>> 2. I'm not 100% sure that unmounting the drive, powering it off,
>>>    removing it, and putting a new disk in its place is legit.  Can
>>>    anyone confirm?  My motherboard *does* claim to support SATA
>>>    hotswap, but I'm not sure if Linux supports it.
>>
>> Linux probably doesn't, save in the most recent kernels, and possibly
>> only with appropriate patches.  
>
> In my case I'm wondering what could possibly go wrong?  If the drive
> is completely unmounted before it is powered down and removed, it
> seems as though the OS has no reason to be concerned with how/when I
> plug it in.  Any ideas?

Well, the worst case is that the hardware can short and fry the entire
controller chip, resulting in a dead hard disk, motherboard and
potentially other components.

That is a pretty bad worst case, but not unknown, for pulling hardware
at random.

A much more likely fault is that your controller will get to exercise
those wonderful, poorly tested, error handling paths as it suddenly
discovers a missing device.

That can lead to anything from the controller hanging to a panic when
the error handler turns out to have a bug.  Not nice.


Also, if you don't stop the drive spinning before you pull it then you
have cut power to a disk in rotation.  That necessitates an emergency
stop of the heads, which isn't great for their life.


Now, it /might/ just work, and if you have hot-swap hardware then the
power issues resulting in physical damage are unlikely.  It isn't nice,
and will result in the hardware and the OS believing that a serious
error has just happened.

>>> Lastly, if there's any standard way to automate backup jobs (mounting
>>> disks, rsync or whatever, unmounting, etc.) I'd appreciate a
>>> reference.  I can always use cron scripts but I imagine someone has
>>> probably come up with something better.
>>
>> udev can fire off arbitrary code on insertion of a device.  You can use
>> that to trigger a script that will, basically, do all the work for you.
>
> Is that really what "support for SATA hot-swap" amounts to?

No.  That is the very last bit.  Hot-swap is the bit where the OS,
driver, controller and everything else is *aware* that changes are going
to happen, so they can handle them gracefully.

udev (and hald, and a bunch of other code written on top of those) are
the icing of the cake: when hot-plug works it can react to it sensibly
and do things like configure your new network card, mount your hard
disk, or whatever.

Hot-swap, as such, is all the bits below that which conspire to make it
work.  On a bus like USB this is well tested, while SATA ... isn't.


Sorry if that wasn't clear to you -- the driver, OS and controller
hardware need to be hot-swap capable for this to have a chance of
working even remotely reliably.

Regards,
        Daniel
-- 
Digital Infrastructure Solutions -- making IT simple, stable and secure
Phone: 0401 155 707        email: contact at digital-infrastructure.com.au
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