[ale] Raid perf?

Greg Freemyer greg.freemyer at gmail.com
Fri Oct 24 18:22:56 EDT 2008


Jeff,

Raid 10 is slightly less redundant than Raid 6

If I have a Raid 10 with 6 drives and one fails I end up with 5 good
drives, but 1 of them is no longer redundant, so if it happened to
fail you lose the whole thing.

(Yes I'm talking about a dual disk failure where both disks happen to
be partners of each other.)

Raid 6 can function in dual drive failure, regardless of which 2 drives fail.

FYI: Raid 10 with large numbers of disks can handle even more than 2
failures if you don't lose any mirror pairs.  Raid 6 will always fail
if you have a 3 disk failure.

OTOH,
Raid 10 has a 50% waste of drives.

Raid 6 is only wasting 2 drives, so if you have 5 or more drives in
your array it has less waste.

The negative is the write speed.  To update a Raid 10, you just have
to write your update to two places.  i.e 2 i/o's per write.

With Raid 6, you apparently have to read the 3 drives worth of info to
figure out the original parity state, then write all 3 of those drives
with updated info.  Thus 6 i/o's for each write.


Greg

On Fri, Oct 24, 2008 at 5:58 PM, Jeff Lightner <jlightner at water.com> wrote:
> Other than performance is the fact that RAID10 can actually suffer TWO
> disk failures before the raid set goes away.   Haven't worked with RAID6
> so don't know its redundancy.
>
> -----Original Message-----
> From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Greg
> Freemyer
> Sent: Friday, October 24, 2008 2:52 PM
> To: Atlanta Linux Enthusiasts
> Subject: Re: [ale] Raid perf?
>
> All,
>
> If anyone cares I found this Dell doc:
>
> http://www.dell.com/downloads/global/products/pvaul/en/PERC6_PerfWP_WMD1
> 120.pdf
>
> Per it, Raid 6 is in theory 3 times less efficient than Raid 10 on
> writes.
>
> So to maintain the same write performance as my 4-disk Raid 10 I would
> need a 12-disk Raid 6.
>
> That is worse than I realized, but feasible for my needs.  (I think
> 250GB drives are down to $60 or so.  Thus 12 is only $700.  Obviously
> I need the controller card too.  Haven't priced that yet.  I do have
> big chassis that can hold the drives.)
>
> Time for me to do a better job of characterizing my app to see what
> percentage the i/o's are reads vs. writes.
>
> Greg
>
> On Fri, Oct 24, 2008 at 9:53 AM, Greg Freemyer <greg.freemyer at gmail.com>
> wrote:
>> All,
>>
>> I have a Raid 10 (4x500GB) under Fedora using software raid that I
>> want to replace with a Raid 6 (6x1TB).
>>
>> Does anyone know a website that will help me understand what the
>> impact will be from a write perspective?  Assuming random DB style
>> writes, not sequential.
>>
>> I'm curious about both theoretical and real world  (3ware or linux
>> software raid) performance numbers.
>>
>> Thanks
>> Greg
>> --
>> Greg Freemyer
>> Litigation Triage Solutions Specialist
>> http://www.linkedin.com/in/gregfreemyer
>> First 99 Days Litigation White Paper -
>>
> http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pd
> f
>>
>> The Norcross Group
>> The Intersection of Evidence & Technology
>> http://www.norcrossgroup.com
>>
>
>
>
> --
> Greg Freemyer
> Litigation Triage Solutions Specialist
> http://www.linkedin.com/in/gregfreemyer
> First 99 Days Litigation White Paper -
> http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pd
> f
>
> The Norcross Group
> The Intersection of Evidence & Technology
> http://www.norcrossgroup.com
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://mail.ale.org/mailman/listinfo/ale
> ----------------------------------
> CONFIDENTIALITY NOTICE: This e-mail may contain privileged or confidential information and is for the sole use of the intended recipient(s). If you are not the intended recipient, any disclosure, copying, distribution, or use of the contents of this information is prohibited and may be unlawful. If you have received this electronic transmission in error, please reply immediately to the sender that you have received the message in error, and delete it. Thank you.
> ----------------------------------
>
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://mail.ale.org/mailman/listinfo/ale
>



-- 
Greg Freemyer
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
First 99 Days Litigation White Paper -
http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pdf

The Norcross Group
The Intersection of Evidence & Technology
http://www.norcrossgroup.com


More information about the Ale mailing list