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

[Matthew Marlowe]

I supported about 250 gentoo vm's using about 30 SAS 15K rpm 144GB
drives awhile back.  Drives were split into 14 disk RAID10 sets.  Then
each RAID10 set was split it into 200-500GB virtual drives, and the
virtual machines were grouped into sets of 3-5 and matched with a
virtual drive.  Virtual machines on the same virtual drive were setup
to use thin provisioning, so that only used up as much storage space
as their data differed from the canonical gentoo os image which was
usually less than 20%.  The virtual drives were usually only 30-50%
full and we could virtually provision 2TB+ of virtual machines on a
single 500GB virtual drive.

Don't underestimate what you can do with small drives, especially if
they are fast and you have a lot of them....

On Thu, Jun 27, 2013 at 5:51 PM, Duncan <1i5t5.duncan@×××.net> wrote:
> 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
>
>