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SCSI reserve/release commands

Posted on 2006-11-14
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I need to know the details of the Reserve and Release scsi commands,
I mean, what exactly they do and how they do it?
And the same about persistent reserve/release command, for what they exist and what is the difference between persistent and the normal reserve/release?
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Question by:tomerlei
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by:adg080898
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See http://www.t10.org/

This site has everything you could ever want to know about scsi protocols.

A short explanation:

SCSI allows devices to be shared on the SCSI bus - you could have multiple computers connected to a single SCSI chain. The reserve and release commands allow a computer to "own" a device and prevent other computers from affecting the device.

For example, say you had two pc's and a scsi burner all on the same scsi bus. One machine might "reserve" the burner device so the other computer won't mess up the burning process.
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by:tomerlei
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what about the persistent reserve/release?
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by:adg080898
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I'm pretty sure persistent reserve/release means "remember across resets". To reserve permanently until release even if power is disconnected. I expect most devices won't support this, unless they have some sort of storage. For example, a hard drive could easily support persistent reserve because it has storage, but, a dvd burner or printer probably won't support it because they usually don't have any non-volatile storage.

A hard drive used to boot the operating system would need persistent reserve because it is critical for it to have exclusive access for the boot process - boot would fail if someone else had it reserved. A printer would rarely need persistent reserve because it is a non-critical device - the OS doesn't require the printer for correct operation.

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Here's something...

http://www.t10.org/ftp/t10/document.95/95-229r2.pdf

Quote from pdf file above:

This proposal extends the definition of reservations to allow proper behavior in multi-initiator and multi-port environments. The proposal defines persistent
reservations which remain valid across Target Reset and can only be cleared by power down or by a properly qualified persistent reservation from another
initiator. Using the commands defined by this proposal, a host can protect the logical unit from improper behavior caused by another initiator on the same or other ports. At the same time, the host can determine from a logical unit which initiators share the logical unit, which initiator is presently reserving the logical unit, and can choose to displace the reservation of an initiator which is known to
have failed.

End quote.

Note that an "initiator" is something that controls the device, almost always a computer. "Multi-initiator" refers to having multiple computers on a single scsi bus.
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