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Automatic bypass of offline network expanders Disclosure Number: IPCOM000244385D
Publication Date: 2015-Dec-08
Document File: 3 page(s) / 53K

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The Prior Art Database


Automatic bypass of offline SAS expanders via powered switches

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This is the abbreviated version, containing approximately 54% of the total text.

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Automatic bypass of offline network expanders

Disclosed is a method for reducing the impact of a SAS expander outage on the overall SAS fabric by means of a powered switch automatically rerouting communications away from the offline expander.

    A typical SAS network consists of an initiator device connected to one or more endpoints via intermediate expander devices. Reaching an endpoint may involve travel through many expanders, and the loss of one expander results in loss of access to all endpoints routed through it, even if not directly attached to that expander.

    An expander may be lost unexpectedly to hardware failure or power failure, or suffer an expected outage (e.g. due to firmware update) or even be excluded deliberately by the initiator device because it or an endpoint connected to it is misbehaving and causing issues for other objects on the network. In order to reduce the impact on the network, it would be advantageous for communications that would otherwise be routed to that expander to be automatically routed to the next expander in a chain during such outage situations, so that only endpoints directly attached to that expander were lost.

Each expander is usually contained within a larger canister, which contains

an in port and an out port, both routed into the expander. Normally the in port and out port are routed directly to the expander via inflexible paths. In this enhancement, a powered, electronically controllable switch is added into the canister, outside of the expander. This may be routing both ports into the expander (active mode) or routing the in port directly to the out port, completely bypassing the expander. (Passthrough mode.)

    When unpowered, this switch defaults to passthrough mode. When the expander has initialised and is ready to start routing IO, it signals the switch to switch to active mode. After this, the switch remains in active mode until another stimulus causes it to change back into passthrough mode.

    Although there are only 2 physical states, management is simplified by subdividing the passthrough state into 3 separate logical states; a powered off state, an auto passthrough state where the switch is in passthrough mode due to loss of contact with the expander, and a manual passthrough state where the expander has explicitly placed the switch into passthrough mode.

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    In an unpowered canister, the switch will be in power off passthrough state. Once power is restored, it will move to the auto passthrough state, where it will wait

until the expander signals that it has started up and is ready to accept IO. This will move it to active state. From there, a loss of communications with the expander will move the switch back to auto passthrough state.

    The expander may put the switch into manual passthrough state if it knows that it will imminently lose the ability to route traff...