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We have an iSCSI SAN setup with Dell's EqualLogic PS series SANs. SAN HQ reports a 30ms write delay. Is this 30ms for the SAN box itself to write the data to the disks, or does the 30ms also include network time getting the data to the SAN box?

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up vote 3 down vote accepted

Those metrics only include "in-box" latency, as the SAN has no way of knowing how long packets took to get there from the HBA. To get total latency numbers, you need to grab metrics from the OSes using the SAN.

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This may be true for write latency, but keep in mind that for read latency SAN HQ factors in the TCP/IP "ACK" responses it receives back from the host, which definitely accounts for network latency. Network congestion usually shows itself in read latency; high write latency is usually more of a sign that the array is getting pushed toward its maximum workload capacity, or there may be a problem with the built-in write caching on the controllers. – JimNim Sep 6 '13 at 14:48

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