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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] LU access through an iSCSI sessionJulo, Thinking about what could be an implementation of an iSCSI session, several questions came to my mind for which i was unable to find an answer in the draft. 1) Do commands directed to a particular LU through a session are able to use any of the TCP connections within the session? As the TCP connections could connect to various "service delivery ports" it is not guarantee that a particular LU is visible through all the connections. 2) Do commands directed to a particular LU through a session need to be associated to different LUNs depending on the TCP connection used or is it always the same LUN for all the connections? As the TCP connections could connect to various "service delivery ports" of the target, it seems that the LUN could differ depending on the connection used. In the iSCSI draft in 2.2.1 there is: "The group of TCP connections linking an initiator with a target forms a session (loosely equivalent to a SCSI nexus)." Does it means that the same LUN is used across the TCP connections? Depending on the answer to these questions the failover/load balancing can be handled in different ways: -> If the answer for 1) is "All TCP connections can be used" and the answer for 2) is "Same LUN regardless of the connexion" For fail over and load balancing inside a session, there is no need to work at the (thin) LU granularity, working at the granularity of the TCP connexion is sufficient. For example in case of failed TCP connexion all the traffic for this failed TCP connexion can be directed on the other connections regardless of the LU. It is very quick, straight forward and may add value compared to the legacy solutions that are doing the fail over on a LU basis (one LU command returns an error or timeout, the next commands for this LU will use an alternate path) In the case where a large number (N) of LUs access are multiplexed to a target port, N switch over are needed. The same result could be achieved in one operation (TCP connection switch over). -> If the answer for 1) is "All TCP connections can be used" and the answer for 2) is "different LUN depending on the connexion" We need to work at the LU granularity. For each command the right LUN as to be determined depending on the tcp connection selected. -> If the answer for 1) is "not all TCP connections can be used, it is LU dependent" On top of the previous case extra work, for each LU it must be a list of TCP connections usable that needs to be looked into before routing the command to one TCP connection. It means that some LUs access using a session could survive to a TCP connection failure some other LUs no. Regards, Pierre
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