> The RAID array is 16 devices attached to a port expander in turn attached to > a SAS controller. At a most simplistic level, surely the SAS controller has > overhead attached to which drive is being addressed. Don't forget that with a port expander is still limited to the bus speed of the link between it and the HBA. It doesn't matter how many drives you hang off an expander, you will still never exceed the rated speed (1.5/3/6Gb/s) for that one port on the HBA. Say you have four drives behind an expander on a 6Gb/s link. Each drive in the array could still bonnie++ at the full 6Gb/s but once you try to write to all four drives simultaneously (RAID-5/6), the best you can get out of each is around 1.5Gb/s. That's why I don't use expanders except for archival SATA drives, which AFAIK only go one expander deep. The performance penalty isn't worth the cost savings in my books. -- Drew "Nothing in life is to be feared. It is only to be understood." --Marie Curie -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
