Re: letting glabel recognise a media change

On 10/10/2010, at 22.55, Alexander Motin wrote:

> Pawel Jakub Dawidek wrote:
>> On Thu, Sep 30, 2010 at 08:46:11PM +0300, Alexander Motin wrote:
>>> Andriy Gapon wrote:
>>>> on 30/09/2010 01:28 Matthew Jacob said the following:
>>>>> If something like that was in place, I assure you that things would start to use
>>>>> it very quickly.
>>>> I am not sure about this.
>>>> Because, e.g. I don't see an easy way to know that media is changed in scsi_cd
>>>> driver.  That is, without polling.  I don't consider polling to be an easy way for
>>>> a number of reasons.
>>> SATA specification defines concept of Asynchronous Notification. It is
>>> already used by port multipliers to report about PHY events. It is also
>>> supposed to be used by CD drives to report media change. I haven't seen
>>> such devices yet, but hope they may appear sometimes.
>>> 
>>> And even without AN support it would be nice to implement proper
>>> handling for SCSI "UA - media changed" errors within CAM. It still won't
>>> be perfect without using polling, but probably still something.
>> 
>> I'd like to know the original reason why CD device is represented by
>> GEOM provider and not CD media. For my naive thinking CD media should be
>> GEOM provider that we taste once the media is inserted and orphan once
>> the media is removed. I don't see any reasons for CD device to be useful
>> GEOM provider, but maybe I'm overlooking something.
>> 
>> Poul-Henning or Soren, do you remember who made and why this design choice?
> 
> SCSI drivers receive no notification on media insertion. Somebody should
> poll device. At this moment it is handled by consumers. Indeed not sure
> it is the best idea. If we want driver to bother with this polling - we
> may do as you propose. Actually in sdhci(4) driver I am doing this way -
> mmcsdX device appears when card inserted and disappears on removal.
> 
> I think first thing that may brake if there will be no cdX device -
> loading the media with some commands. Also it may be non-intuitive that
> drive is present, but disk is not registered in GEOM.

Think burning data on empty media.

-Søren

"Any sufficiently advanced technology is indistinguishable from magic."
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