Re: Linux Cluster using shared scsi

[James Bottomley <James.Bottomley@steeleye.com>]

Eric.Ayers@intec-telecom-systems.com said:
> Does this package also tell the kernel to "re-establish" a reservation
> for all devices after a bus reset, or at least inform a user level
> program?  Finding out when there has been a bus reset has been a
> stumbling block for me. 

alan@lxorguk.ukuu.org.uk said:
> You cannot rely on a bus reset. Imagine hot swap disks on an FC
> fabric. I  suspect the controller itself needs to call back for
> problem events 

Essentially, there are many conditions which cause a quiet loss of a SCSI-2 
reservation.  Even in parallel SCSI: Reservations can be silently lost because 
of LUN reset, device reset or even simple powering off the device.

The way we maintain reservations for LifeKeeper is to have a user level daemon 
ping the device with a reservation command every few minutes.  If you get a 
RESERVATION_CONFLICT return you know that something else stole your 
reservation, otherwise you maintain it.  There is a window in this scheme 
where the device may be accessible by other initiators but that's the price 
you pay for using SCSI-2 reservations instead of the more cluster friendly 
SCSI-3 ones.  In a kernel scheme, you may get early notification of 
reservation loss by putting a hook into the processing of 
CHECK_CONDITION/UNIT_ATTENTION, but it won't close the window entirely.

James