Re: [PATCH 1/3] sd: don't bother spinning up disks on resume

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On 11/17/2013 06:54 PM, Douglas Gilbert wrote:
> Even if the SCSI EH code does spin-up the disk, it is inefficient
> and undesirable to send a device into EH for what is a normal,
> predictable situation (i.e. that a SCSI disk will need a START STOP
> UNIT (start) command as part of a resume operation).

The device doesn't need the START STOP UNIT command as part of the
resume operation; it only needs it prior to other commands.  What is
inefficient is starting up drives that won't be accessed.  I don't see
anything particularly inefficient about issuing the command in the eh
path ( in fact, libata always uses the eh path to handle power
management ).  You can of course, use the manage_start_stop flag to
have the disk start on resume if you really want.

> A big server machine could have thousands of SCSI disks (many
> potentially virtual). Sending them all into EH during a resume
> would be a really good test for EH, but horrible for performance.
> If I was designing the SCSI EH I would probably assume not all
> disks would go into EH at roughly the same time. Also if that many
> devices went into EH on the same controller (HBA) it might be
> reasonable to assume that the controller they share needs
> resetting.

They don't all go into EH during resume; they go into EH when they are
finally accessed, which for many of them is probably not for some
time.  This also sounds like a bit of speculation and/or hand waving.
 Is there really a problem with multiple devices going into EH?


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