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

[Duncan <1i5t5.duncan@cox.net>]

Mark Knecht posted on Sat, 22 Jun 2013 16:04:06 -0700 as excerpted:

> Lastly, even if I completely buy into Duncan's well formed reasons about
> why RAID1 might be faster, using 500GB drives I see no single RAID
> solution for me other than RAID5/6. The real RAID1/RAID6 comparison from
> a storage standpoint would be a (conceptual) 3-drive RAID6 vs 3 drive
> RAID1. Both create 500GB of storage and can (conceptually) lose 2 drives
> and still recover data. However adding another drive to the RAID1 gains
> you more speed but no storage (buying into Duncan's points) vs adding
> storage to the RAID6 and probably reducing speed. As I need storage what
> other choices do I have?
> 
>    Answering myself, take the 5 drives, create two RAIDS - a 500GB
> 2-drive RAID1 for the system + VMs, and then a 3-drive RAID5 for video
> data maybe? I don't know...
> 
>    Or buy more hardware and do a 2 drive SSD RAID1 for the system, or
> a hardware RAID controller, etc. The options explode if I start buying
> more hardware.

Finally getting back to this on what's my "weekend"...

Unfortunately, given 900 gigs media data and 150 gigs of VMs, with 5 500 
gig drives to work with, you're right, simply making a raid1 out of 
everything isn't possible.

You could do a 4-drive raid10, two-way striped and two-way mirrored, for 
a TB of storage for the media files and possibly squeeze the VMs between 
the SSD and the raid, with the 5th half-TB as a backup, but it'd be quite 
tight and non-optimal, plus losing the wrong two drives on the raid10 
would put it out of commission so you'd have only one-drive-loss-
tolerance there.

You could buy a sixth half-TB and try either three-way-striping and two-
way mirroring for the same one-drive-loss tolerance but a good 1.5 TB (3-
way half-TB stripe) space, giving you plenty of space and thruput speed 
but at the cost of only single-drive-loss-tolerance.

You could use the same six in a raid10 with the reverse configuration, 
two-way-stripe three-way-mirror, for better loss-of-two-tolerance but at 
only a TB of space and have the same squeeze as the 4-way raid10 (but now 
without the extra drive for backup), or...

Personally, I'd probably be intensely motivated enough to try the 2-way-
stripe 3-way-mirror 6-drive raid10, squeezing the media space as 
necessary to do it (maybe by using external drives for what wouldn't 
fit), but that's still a compromise... and includes buying that sixth 
drive.

So the raid6 might well be the best alternative you have, given the data 
size AND physical device size constraints.

But some time testing the performance of different configs and 
familiarizing yourself with the options and operation, as you've decided 
to do now, certainly won't hurt.  I DID say I wasn't real strong on the 
chunk options, etc, myself, and you're using ext4, not the reiserfs I was 
using, and I believe ext4 has at least some potential performance upside 
compared to reiserfs, so it's quite possible that with some chunk/stride/
etc tweaking, you can get something better, performance-wise.  Tho I 
expect raid6 will never be a speed demon, and may well never perform as 
you had originally expected/hoped.  But better than the initial results 
should be possible, hopefully, and familiarizing yourself with things 
while experimenting has benefits of its own, so that's an idea I can 
agree with 100%. =:^)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman