Gvinum RAID5 performance

[Greg 'groggy' Lehey]

On Sunday, 31 October 2004 at 23:56:17 +0100, Lukas Ertl wrote:
> On Mon, 1 Nov 2004 freebsd at newmillennium.net.au wrote:
>
>> Now, running a dd from a plex gives me less performance than running a
>> dd from one of the subdisks, even though the array is not running in
>> degraded mode.
>
> I'd consider this normal behaviour, since you have to go through the
> offset calculation.

The offset calculations should run at several orders of magnitude
faster than the disk transfers.  If reading a plex is slower than
reading the subdisk, I can see two reasons:

1.  Too small a stripe size.  If you (our anonymous user, who was
    using a single dd process) have to perform multiple transfers for
    a single request, the results will be slower.
2.  There may be some overhead in GEOM that slows things down.  If
    this is the case, something should be done about it.

I'm pretty sure that the old Vinum would not show this kind of
behaviour if (1) doesn't apply.

Greg
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