followup storage question
Chad J. Milios
milios at ccsys.com
Fri Sep 11 16:10:21 UTC 2015
> On Sep 11, 2015, at 11:02 AM, William A. Mahaffey III <wam at hiwaay.net> wrote:
> Hmmmm .... OK, however if I carefully 'gpart -a 4k' the underlying partitions aligned, then wouldn't the resulting ZFS above also be aligned ?
Thanks to gpart's -a setting the partition will be a multiple of 4k in size and aligned upon the outer context (disk) at 4k boundaries but the alignment/size of [each/every] i/o request that ZFS sends to the device node, well that is a different story. That is what the ashift value controls and it gets stamped onto the new vdev when ZFS first christens a device/partition. The sysctl or gnop helps us ensure the ashift value we bake into a vdev is truly the value we want after all the low down dirty deceitful layers of abstraction beneath that are often lying to us.
The situation's no better on any other operating system, even if they hide it better that only opens up the possibility it makes wrong guesses. This is just a growing pain resulting from ever increasing disk/file sizes/densities vs the demand for backward compatibility.
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