Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Rich Freeman <rich0@gentoo.org>]

On Fri, Jun 21, 2013 at 1:40 PM, Mark Knecht <markknecht@gmail.com> wrote:
>    One place where I wanted to double check your thinking. My thought
> is that a RAID1 will _NEVER_ outperform the hdparm -tT read speeds as
> it has to read from three drives and make sure they are all good
> before returning data to the user.

That isn't correct.   In theory it could be done that way, but every
raid1 implementation I've heard of makes writes to all drives
(obviously), but reads from only a single drive (assuming it is
correct).  That means that read latency is greatly reduced since they
can be split across two drives which effectively means two heads per
"platter."  Also, raid1 typically does not include checksumming, so if
there is a discrepancy between the drives there is no way to know
which one is right.  With raid5 at least you can always correct
discrepancies if you have all the disks (though as Duncan pointed out
in practice this only happens if you do an explicit scrub on mdadm).
With btrfs every block is checksummed and so as long as there is one
good (err, consistent) copy somewhere it will be used.

Rich