Re: [PATCH 4/4 v2] Add documentation for hard disk shock protection interface

["Grant Grundler" <grundler@google.com>]

I found one typo here...can be fixed up by hand I think.

On Wed, Sep 17, 2008 at 9:40 AM, Elias Oltmanns <eo@nebensachen.de> wrote:
> Put some information (and pointers to more) into the kernel's doc tree,
> describing briefly the interface to the kernel's disk head unloading
> facility. Information about how to set up a complete shock protection
> system under GNU/Linux can be found on the web and is referenced
> accordingly.
>
> Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
> ---
>
>  Documentation/laptops/disk-shock-protection.txt |  144 +++++++++++++++++++++++
>  1 files changed, 144 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/laptops/disk-shock-protection.txt
>
> diff --git a/Documentation/laptops/disk-shock-protection.txt b/Documentation/laptops/disk-shock-protection.txt
> new file mode 100644
> index 0000000..1f93462
> --- /dev/null
> +++ b/Documentation/laptops/disk-shock-protection.txt
> @@ -0,0 +1,144 @@
> +Hard disk shock protection
> +==========================
> +
> +Author: Elias Oltmanns <eo@nebensachen.de>
> +Last modified: 2008-09-16
> +
> +
> +0. Contents
> +-----------
> +
> +1. Intro
> +2. The interface
> +3. References
> +4. CREDITS
> +
> +
> +1. Intro
> +--------
> +
> +ATA/ATAPI-7 specifies the IDLE IMMEDIATE command with unload feature.
> +Issuing this command should cause the drive to switch to idle mode and
> +unload disk heads. This feature is being used in modern laptops in
> +conjunction with accelerometers and appropriate software to implement
> +a shock protection facility. The idea is to stop all I/O operations on
> +the internal hard drive and park its heads on the ramp when critical
> +situations are anticipated. The desire to have such a feature
> +available on GNU/Linux systems has been the original motivation to
> +implement a generic disk head parking interface in the Linux kernel.
> +Please note, however, that other components have to be set up on your
> +system in order to get disk shock protection working (see section
> +3. References below for pointers to more information about that).

Unfortunate place for a line break: "section 3" should be on one line if
possible. Otherwise the "3. References" looks like section header with
mangled white space (at first glance).


> +
> +
> +2. The interface
> +----------------
> +
> +For each ATA device the kernel exports the file
> +block/*/device/unload_heads in sysfs (here assumed to be mounted under
> +/sys). Access to /sys/block/*/device/unload_heads is denied with
> +-EOPNOTSUPP if the device does not support the unload feature.
> +Otherwise, writing an integer value to file will take the heads of the
> +respective drive off the platter and block all I/O operations for the
> +specified number of milliseconds. When the timeout expires and no
> +further disk head park request has been issued in the meantime, normal
> +operation will be resumed. The maximal value accepted for a timeout is
> +30000 milliseconds. Exceeding this limit will return -EOVERFLOW, but
> +heads will be parked anyway and the timeout will be set to 30 seconds.
> +However, you can always change a timeout to any value between 0 and
> +30000 by issuing a subsequent head park request before the timeout of
> +the previous one has expired. In particular, the total timeout can
> +exceed 30 seconds and, more importantly, you can cancel a previously
> +set timeout and resume normal operation immediately by specifying a
> +timeout of 0. Values below -2 are rejected with -EINVAL (see below for
> +the special meaning of -1 and -2). If the timeout specified for a
> +recent head park request has not yet expired, reading from
> +/sys/block/*/device/unload_heads will report the number of
> +milliseconds remaining until normal operation will be resumed;
> +otherwise, reading the unload_heads attribute will return 0.
> +
> +For example, do the following in order to park the heads of drive
> +/dev/sda and stop all I/O operations for five seconds:
> +
> +# echo 5000 > /sys/block/sda/device/unload_heads
> +
> +A simple
> +
> +# cat /sys/block/sda/device/unload_heads
> +
> +will show you how many milliseconds are left before normal operation
> +will be resumed.
> +
> +There is a technical detail of this implementation that may cause some
> +confusion and should be discussed here. When a head park request has
> +been issued to a device successfully, all I/O operations on the
> +controller port this device is attached to will be deferred. That is
> +to say, any other device that may be connected to the same port will
> +be affected too. The only exception is that a subsequent head unload
> +request to that other devvice will be executed immediately. Further
> +operations on that port will be deferred until the timeout specified
> +for either device on the port has expired. As far as PATA (old style
> +IDE) configurations are concerned, there can only be two devices
> +attached to any single port. In SATA world we have port multipliers
> +which means that a user issued head parking request to one device may
> +actually result in stopping I/O to a whole bunch of devices. Hwoever,

Typo: However

> +since this feature is supposed to be used on laptops and does not seem
> +to be very useful in any other environment, there will be mostly one
> +device per port. Even if the CD/DVD writer happens to be connected to
> +the same port as the hard drive, it generally *should* recover just
> +fine from the occasional buffer under-run incurred by a head park
> +request to the HD. Actually, when you are using an ide driver rather
> +than it's libata counterpart (i.e. your disk is called /dev/hda
> +instead of /dev/sda), then parking the heads of drive A will generally
> +not affect the mode of operation of drive B on the same port as
> +described above. It is only when a port reset is required to recover
> +from an exception on drive B that further I/O operations on that drive
> +(and the reset itself) will be delayed until drive A is no longer in
> +the parked state.
> +
> +Finally, there are some hard drives that only comply with an earlier
> +version of the ATA standard than ATA-7, but do support the unload
> +feature nonetheless. Unfortunately, there is no safe way Linux can
> +detect these devices, so you won't be able to write to the
> +unload_heads attribute. If you know that your device really does
> +support the unload feature (for instance, because the vendor of your
> +laptop or the hard drive itself told you so), then you can tell the
> +kernel to enable the usage of this feature for that drive by writing
> +the special value -1 to the unload_heads attribute:
> +
> +# echo -1 > /sys/block/sda/device/unload_heads
> +
> +will enable the feature for /dev/sda, and giving -2 instead of -1 will
> +disable it again.
> +
> +
> +3. References
> +-------------
> +
> +There are several laptops from different vendors featuring shock
> +protection capabilities. As manufacturers have refused to support open
> +source development of the required software components so far, Linux
> +support for shock protection varies considerably between different
> +hardware implementations. Ideally, this section should contain a list
> +of pointers at different projects aiming at an implementation of shock
> +protection on different systeems. Unfortunately, I only know of a
> +single project which, although still considered experimental, is fit
> +for use. Please feel free to add projects that have been the victims
> +of my ignorance.
> +
> +- http://www.thinkwiki.org/wiki/HDAPS
> +  See this page for information about Linux support of the hard disk
> +  active protection system as implemented in IBM/Lenovo Thinkpads.
> +  (FIXME: The information there will have to be updated once this
> +  patch has been approved or the user interface has been agreed upon
> +  at least.)

Does this still belong here?

> +
> +
> +4. CREDITS
> +----------
> +
> +This implementation of disk head parking has been inspired by a patch
> +originally published by Jon Escombe <lists@dresco.co.uk>. My efforts
> +to develop an implementation of this feature that is fit to be merged
> +into mainline have been aided by various kernel developers, in
> +particular by Tejun Heo and Bartlomiej Zolnierkiewicz.

otherwise looks pretty good - thanks!
grant

>
>
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