ata: Add documentation for hard disk shock protection interface (v3)

Hard disk shock protection

==========================

Author: Elias Oltmanns <eo@nebensachen.de>

Last modified: 2008-10-03

0. Contents

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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).

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 this 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.

A word of caution: The fact that the interface operates on a basis of

milliseconds may raise expectations that cannot be satisfied in

reality. In fact, the ATA specs clearly state that the time for an

unload operation to complete is vendor specific. The hint in ATA-7

that this will typically be within 500 milliseconds apparently has

been dropped in ATA-8.

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 device 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. 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 its libata counterpart (i.e. your disk is called /dev/hda

instead of /dev/sda), then parking the heads of one drive (drive X)

will generally not affect the mode of operation of another drive

(drive Y) on the same port as described above. It is only when a port

reset is required to recover from an exception on drive Y that further

I/O operations on that drive (and the reset itself) will be delayed

until drive X 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 systems. 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.

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.