What RAID type and why?

What RAID type and why?

[Mark Knecht]

First post. I've never used RAID but am thinking about it and looking
for newbie-level info. Thanks in advance.

I'm thinking about building a machine for long term number crunching
of stock market data. Highest end processor I can get, 16GB and at
least reasonably fast drives. I've not done RAID before and don't know
how to choose one RAID type over another for this sort of workload.
All I know is I want the machine to run 24/7 computing 100% of the
time and be reliable at least in the sense of not losing data if 1
drive or possibly 2 go down.

If a drive does go down I'm not overly worried about down time. I'll
stock a couple of spares when I build the machine and power the box
back up within an hour or two.

What RAID type do I choose and why?

Do I need a 5 physical drive RAID array to meet these requirements?
Assume 1TB+ drives all around.

How critical is it going forward with Linux RAID solutions to be able
to get exactly the same drives in the future? 1TB today is 4TB a year
from now, etc.

With an 8 core processor (high-end Intel Core i7 probably) do I need
to worry much about CPU usage doing RAID? I suspect not and I don't
really want to get into hardware RAID controllers unless critically
necessary which I suspect it isn't.

Anyway, if there's a document around somewhere that helps a newbie
like me I'd sure appreciate finding out about it.

Thanks,
Mark

Re: What RAID type and why?

[Greg Freemyer]

On Sat, Mar 6, 2010 at 5:02 PM, Mark Knecht <markknecht@gmail.com> wrote:
> First post. I've never used RAID but am thinking about it and looking
> for newbie-level info. Thanks in advance.
>
> I'm thinking about building a machine for long term number crunching
> of stock market data. Highest end processor I can get, 16GB and at
> least reasonably fast drives. I've not done RAID before and don't know
> how to choose one RAID type over another for this sort of workload.
> All I know is I want the machine to run 24/7 computing 100% of the
> time and be reliable at least in the sense of not losing data if 1
> drive or possibly 2 go down.
>
> If a drive does go down I'm not overly worried about down time. I'll
> stock a couple of spares when I build the machine and power the box
> back up within an hour or two.
>
> What RAID type do I choose and why?
>
> Do I need a 5 physical drive RAID array to meet these requirements?
> Assume 1TB+ drives all around.
>
> How critical is it going forward with Linux RAID solutions to be able
> to get exactly the same drives in the future? 1TB today is 4TB a year
> from now, etc.
>
> With an 8 core processor (high-end Intel Core i7 probably) do I need
> to worry much about CPU usage doing RAID? I suspect not and I don't
> really want to get into hardware RAID controllers unless critically
> necessary which I suspect it isn't.
>
> Anyway, if there's a document around somewhere that helps a newbie
> like me I'd sure appreciate finding out about it.
>
> Thanks,
> Mark

I'm not sure about a newbie doc, but here's some basics:

You haven't said what kind of i/o rates you expect, nor how much
storage you need.

At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
which may be a problem.

Raid-5 is out of favor for me due to issues people are seeing with
discrete bad sectors with the remaining drives after you have a drive
failure.  raid-6 tolerates those much better.  Even raid 10 is not as
robust as raid 6 and with the current generation drives robustness in
the raid solution is more important than ever.

But raid 6 uses 2 parity drives, so you'll only get 1TB of useable
space from a 3-disk raid 6 made from 1TB drives.

mdraid just requires replacement disks be bigger than the old disk
you're replacing.

You might consider layering LVM on top of mdraid to help you manage
the array as it grows.

Greg
-- 
Greg Freemyer
Head of EDD Tape Extraction and Processing team
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
Preservation and Forensic processing of Exchange Repositories White Paper -
<http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html>

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

Re: What RAID type and why?

[Mark Knecht]

On Sat, Mar 6, 2010 at 2:33 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
> On Sat, Mar 6, 2010 at 5:02 PM, Mark Knecht <markknecht@gmail.com> wrote:
>> First post. I've never used RAID but am thinking about it and looking
>> for newbie-level info. Thanks in advance.
>>
>> I'm thinking about building a machine for long term number crunching
>> of stock market data. Highest end processor I can get, 16GB and at
>> least reasonably fast drives. I've not done RAID before and don't know
>> how to choose one RAID type over another for this sort of workload.
>> All I know is I want the machine to run 24/7 computing 100% of the
>> time and be reliable at least in the sense of not losing data if 1
>> drive or possibly 2 go down.
>>
>> If a drive does go down I'm not overly worried about down time. I'll
>> stock a couple of spares when I build the machine and power the box
>> back up within an hour or two.
>>
>> What RAID type do I choose and why?
>>
>> Do I need a 5 physical drive RAID array to meet these requirements?
>> Assume 1TB+ drives all around.
>>
>> How critical is it going forward with Linux RAID solutions to be able
>> to get exactly the same drives in the future? 1TB today is 4TB a year
>> from now, etc.
>>
>> With an 8 core processor (high-end Intel Core i7 probably) do I need
>> to worry much about CPU usage doing RAID? I suspect not and I don't
>> really want to get into hardware RAID controllers unless critically
>> necessary which I suspect it isn't.
>>
>> Anyway, if there's a document around somewhere that helps a newbie
>> like me I'd sure appreciate finding out about it.
>>
>> Thanks,
>> Mark
>
> I'm not sure about a newbie doc, but here's some basics:
>
> You haven't said what kind of i/o rates you expect, nor how much
> storage you need.

Good points. I guess I was assuming I'd want 1TB storage and I'd buy
3/5/6 1TB drives to get it. Honestly I probably don't need anything
close to that. My weekly backups of stock data run about 1GB to 1TB
should hold me for quite awhile I think.

As for i/o rates I think it's pretty low. Real-time or historic stock
data arrives here over the net so that's not fast. Crunching numbers
*typically* amounts to loading a single data set from disk into memory
and then operating from there so I suspect that even in backtesting
it's pretty low but I'll see if I can get some data. None the less I'm
not sure there's much overlap between when the disk is heavily used
and when it gets CPU limited. Again, I'll have to give that some
thought.

>
> At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> which may be a problem.
>
> Raid-5 is out of favor for me due to issues people are seeing with
> discrete bad sectors with the remaining drives after you have a drive
> failure.  raid-6 tolerates those much better.  Even raid 10 is not as
> robust as raid 6 and with the current generation drives robustness in
> the raid solution is more important than ever.
>
> But raid 6 uses 2 parity drives, so you'll only get 1TB of useable
> space from a 3-disk raid 6 made from 1TB drives.

I've been looking at this page so far for the most basic info:

http://en.wikipedia.org/wiki/RAID#Organization

They show RAID 6 with 5 drives so I'll need to learn how to do this
with fewer drives. I think you're point about more than 1 drive having
problems around the same time is good input.

While money is always important buying 1 or 2 more drives (say $200)
isn't the biggest issue here. It's a new machine with a $500 processor
so if more drives make a big difference in terms of reliability then I
don't want to cut too many corners.

>
> mdraid just requires replacement disks be bigger than the old disk
> you're replacing.
>
> You might consider layering LVM on top of mdraid to help you manage
> the array as it grows.

Two subject I haven't even thought of!

Thanks for the info! Lots to study!

Cheers,
Mark
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Re: What RAID type and why?

[Keld Simonsen]

There is s newbie setup howto at
http://raid.wiki.kernel.org/index.php/Preventing_against_a_failing_disk

This is for 2 disks, but you can add more disks for added redundency and
speed.

best regards
keld

On Sat, Mar 06, 2010 at 03:05:46PM -0800, Mark Knecht wrote:
> On Sat, Mar 6, 2010 at 2:33 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
> > On Sat, Mar 6, 2010 at 5:02 PM, Mark Knecht <markknecht@gmail.com> wrote:
> >> First post. I've never used RAID but am thinking about it and looking
> >> for newbie-level info. Thanks in advance.
> >>
> >> I'm thinking about building a machine for long term number crunching
> >> of stock market data. Highest end processor I can get, 16GB and at
> >> least reasonably fast drives. I've not done RAID before and don't know
> >> how to choose one RAID type over another for this sort of workload.
> >> All I know is I want the machine to run 24/7 computing 100% of the
> >> time and be reliable at least in the sense of not losing data if 1
> >> drive or possibly 2 go down.
> >>
> >> If a drive does go down I'm not overly worried about down time. I'll
> >> stock a couple of spares when I build the machine and power the box
> >> back up within an hour or two.
> >>
> >> What RAID type do I choose and why?
> >>
> >> Do I need a 5 physical drive RAID array to meet these requirements?
> >> Assume 1TB+ drives all around.
> >>
> >> How critical is it going forward with Linux RAID solutions to be able
> >> to get exactly the same drives in the future? 1TB today is 4TB a year
> >> from now, etc.
> >>
> >> With an 8 core processor (high-end Intel Core i7 probably) do I need
> >> to worry much about CPU usage doing RAID? I suspect not and I don't
> >> really want to get into hardware RAID controllers unless critically
> >> necessary which I suspect it isn't.
> >>
> >> Anyway, if there's a document around somewhere that helps a newbie
> >> like me I'd sure appreciate finding out about it.
> >>
> >> Thanks,
> >> Mark
> >
> > I'm not sure about a newbie doc, but here's some basics:
> >
> > You haven't said what kind of i/o rates you expect, nor how much
> > storage you need.
> 
> Good points. I guess I was assuming I'd want 1TB storage and I'd buy
> 3/5/6 1TB drives to get it. Honestly I probably don't need anything
> close to that. My weekly backups of stock data run about 1GB to 1TB
> should hold me for quite awhile I think.
> 
> As for i/o rates I think it's pretty low. Real-time or historic stock
> data arrives here over the net so that's not fast. Crunching numbers
> *typically* amounts to loading a single data set from disk into memory
> and then operating from there so I suspect that even in backtesting
> it's pretty low but I'll see if I can get some data. None the less I'm
> not sure there's much overlap between when the disk is heavily used
> and when it gets CPU limited. Again, I'll have to give that some
> thought.
> 
> >
> > At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> > which may be a problem.
> >
> > Raid-5 is out of favor for me due to issues people are seeing with
> > discrete bad sectors with the remaining drives after you have a drive
> > failure.  raid-6 tolerates those much better.  Even raid 10 is not as
> > robust as raid 6 and with the current generation drives robustness in
> > the raid solution is more important than ever.
> >
> > But raid 6 uses 2 parity drives, so you'll only get 1TB of useable
> > space from a 3-disk raid 6 made from 1TB drives.
> 
> I've been looking at this page so far for the most basic info:
> 
> http://en.wikipedia.org/wiki/RAID#Organization
> 
> They show RAID 6 with 5 drives so I'll need to learn how to do this
> with fewer drives. I think you're point about more than 1 drive having
> problems around the same time is good input.
> 
> While money is always important buying 1 or 2 more drives (say $200)
> isn't the biggest issue here. It's a new machine with a $500 processor
> so if more drives make a big difference in terms of reliability then I
> don't want to cut too many corners.
> 
> >
> > mdraid just requires replacement disks be bigger than the old disk
> > you're replacing.
> >
> > You might consider layering LVM on top of mdraid to help you manage
> > the array as it grows.
> 
> Two subject I haven't even thought of!
> 
> Thanks for the info! Lots to study!
> 
> Cheers,
> Mark
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
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Re: What RAID type and why?

[Matt Garman]

On Sat, Mar 06, 2010 at 03:05:46PM -0800, Mark Knecht wrote:
> On Sat, Mar 6, 2010 at 2:33 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
> > You haven't said what kind of i/o rates you expect, nor how much
> > storage you need.
> 
> Good points. I guess I was assuming I'd want 1TB storage and I'd
> buy 3/5/6 1TB drives to get it. Honestly I probably don't need
> anything close to that. My weekly backups of stock data run about
> 1GB to 1TB should hold me for quite awhile I think.

Where are you sourcing the stock data?  1GB/week seems awful low.
You must be getting top of book only?

We get real-time full depth stock data.  NYSE and Nasdaq data are
each about 100 GB/month, compressed.

Just something to keep in mind if you ever start working with
full-depth feeds.

Also: for backups, you might want to consider par2.  You can use it
to create a specified amount of parity data for each file or group
of files.  Useful in the case of media errors.

Re: What RAID type and why?

[Mark Knecht]

On Mon, May 10, 2010 at 8:20 AM, Matt Garman <matthew.garman@gmail.com> wrote:
> On Sat, Mar 06, 2010 at 03:05:46PM -0800, Mark Knecht wrote:
>> On Sat, Mar 6, 2010 at 2:33 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
>> > You haven't said what kind of i/o rates you expect, nor how much
>> > storage you need.
>>
>> Good points. I guess I was assuming I'd want 1TB storage and I'd
>> buy 3/5/6 1TB drives to get it. Honestly I probably don't need
>> anything close to that. My weekly backups of stock data run about
>> 1GB to 1TB should hold me for quite awhile I think.
>
> Where are you sourcing the stock data?  1GB/week seems awful low.
> You must be getting top of book only?
>
> We get real-time full depth stock data.  NYSE and Nasdaq data are
> each about 100 GB/month, compressed.
>
> Just something to keep in mind if you ever start working with
> full-depth feeds.
>
> Also: for backups, you might want to consider par2.  You can use it
> to create a specified amount of parity data for each file or group
> of files.  Useful in the case of media errors.
>

Yeah, my description was a bit lacking. Sorry.

Currently I only auto-trade index futures so I get only the indexes
(tick & 1 minute) and then a few other general things (VIX, A/D, Gold,
Oil, 3 month T bill, etc.) for correlation purposes. I don't mess much
with individual stocks at all although I have it in my mind to start
creating some private out of individual stock data so if I do that
storage requirements will certainly go up.

Thanks,
Mark
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RE: What RAID type and why?

[Guy Watkins]

} -----Original Message-----
} From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
} owner@vger.kernel.org] On Behalf Of Greg Freemyer
} Sent: Saturday, March 06, 2010 5:33 PM
} To: Mark Knecht
} Cc: Linux-RAID
} Subject: Re: What RAID type and why?
} 
} On Sat, Mar 6, 2010 at 5:02 PM, Mark Knecht <markknecht@gmail.com> wrote:
} > First post. I've never used RAID but am thinking about it and looking
} > for newbie-level info. Thanks in advance.
} >
} > I'm thinking about building a machine for long term number crunching
} > of stock market data. Highest end processor I can get, 16GB and at
} > least reasonably fast drives. I've not done RAID before and don't know
} > how to choose one RAID type over another for this sort of workload.
} > All I know is I want the machine to run 24/7 computing 100% of the
} > time and be reliable at least in the sense of not losing data if 1
} > drive or possibly 2 go down.
} >
} > If a drive does go down I'm not overly worried about down time. I'll
} > stock a couple of spares when I build the machine and power the box
} > back up within an hour or two.
} >
} > What RAID type do I choose and why?
} >
} > Do I need a 5 physical drive RAID array to meet these requirements?
} > Assume 1TB+ drives all around.
} >
} > How critical is it going forward with Linux RAID solutions to be able
} > to get exactly the same drives in the future? 1TB today is 4TB a year
} > from now, etc.
} >
} > With an 8 core processor (high-end Intel Core i7 probably) do I need
} > to worry much about CPU usage doing RAID? I suspect not and I don't
} > really want to get into hardware RAID controllers unless critically
} > necessary which I suspect it isn't.
} >
} > Anyway, if there's a document around somewhere that helps a newbie
} > like me I'd sure appreciate finding out about it.
} >
} > Thanks,
} > Mark
} 
} I'm not sure about a newbie doc, but here's some basics:
} 
} You haven't said what kind of i/o rates you expect, nor how much
} storage you need.
} 
} At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
} which may be a problem.

If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
and you don't have the RAID6 I/O overhead.

Also, you said your data is important.  If so, you need a backup solution!
2 copies with 1 off-site.  Maybe alternate between the 2 each day or week.

How much data per day?  How much data during the next 3 years?

Guy

} 
} Raid-5 is out of favor for me due to issues people are seeing with
} discrete bad sectors with the remaining drives after you have a drive
} failure.  raid-6 tolerates those much better.  Even raid 10 is not as
} robust as raid 6 and with the current generation drives robustness in
} the raid solution is more important than ever.
} 
} But raid 6 uses 2 parity drives, so you'll only get 1TB of useable
} space from a 3-disk raid 6 made from 1TB drives.
} 
} mdraid just requires replacement disks be bigger than the old disk
} you're replacing.
} 
} You might consider layering LVM on top of mdraid to help you manage
} the array as it grows.
} 
} Greg
} --
} Greg Freemyer
} Head of EDD Tape Extraction and Processing team
} Litigation Triage Solutions Specialist
} http://www.linkedin.com/in/gregfreemyer
} Preservation and Forensic processing of Exchange Repositories White Paper
} -
} <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html>
} 
} The Norcross Group
} The Intersection of Evidence & Technology
} http://www.norcrossgroup.com
} --
} To unsubscribe from this list: send the line "unsubscribe linux-raid" in
} the body of a message to majordomo@vger.kernel.org
} More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: What RAID type and why?

[Mark Knecht]

On Sat, Mar 6, 2010 at 3:17 PM, Guy Watkins <linux-raid@watkins-home.com> wrote:
<SNIP>
> }
> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> } which may be a problem.
>
> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
> and you don't have the RAID6 I/O overhead.
>
> Also, you said your data is important.  If so, you need a backup solution!
> 2 copies with 1 off-site.  Maybe alternate between the 2 each day or week.
>
> How much data per day?  How much data during the next 3 years?
>
> Guy

OK - good points.

The 'data' is very important to me. Full disclosure (and it well could
make a difference I suppose) but the stock data is really just part of
a medium sized VMWare image on the order of 10GB. VMWare is running on
Gentoo and hosting Windows XP currently, Windows 7 later possibly.
Windows is the only platform that has programs that currently do what
I need. (And my trading partner is completely Windows based so until I
convert him to Linux Windows must be part of the recipe.)

Anyway, I try to keep my VMWare images below 10GB so that I can tar
them up once a week and write them to a dual-layer DVD for backup.
Once a month I move one to the safe deposit box at the bank. Otherwise
I keep three in the house at the far end in a fireproof box. (Although
high temp might damage them if the whole house goes up. Who knows...)

So, Gentoo hosts VMWare. VMWare maps a logical 10GB Windows C: drive
into a bunch of 2GB files and the stock data it typically a single
file for each stock where the file size approaches 100MB. I have
little or no control in terms of how that 100MB file is placed on the
drive.

I haven't a clue what that really means in terms of disk access from a
Linux point of view.

Daily backups are just my programming. They are incremental and just
sent to another computer on the network, and then also covered in the
VMWare backup at the end of the week.


Replying to Asdo - I'm hoping to use a more or less standard desktop
or server motherboard with 6-8 SATA ports dedicating 3-6 ports to this
RAID interface and then using Linux software RAID and no hardware RAID
controllers. Is this unreasonable in your opinion for the work flow
I'm describing using VMWare?

I'm concerned about any RAID hardware that leaves me stranded if the
controller dies.

Thanks all!

- Mark
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Re: What RAID type and why?

[Bill Davidsen]

Mark Knecht wrote:
> On Sat, Mar 6, 2010 at 3:17 PM, Guy Watkins <linux-raid@watkins-home.com> wrote:
> <SNIP>
>   
>> }
>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>> } which may be a problem.
>>
>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>> and you don't have the RAID6 I/O overhead.
>>
>> Also, you said your data is important.  If so, you need a backup solution!
>> 2 copies with 1 off-site.  Maybe alternate between the 2 each day or week.
>>
>> How much data per day?  How much data during the next 3 years?
>>
>> Guy
>>     
>
> OK - good points.
>
> The 'data' is very important to me. Full disclosure (and it well could
> make a difference I suppose) but the stock data is really just part of
> a medium sized VMWare image on the order of 10GB. VMWare is running on
> Gentoo and hosting Windows XP currently, Windows 7 later possibly.
> Windows is the only platform that has programs that currently do what
> I need. (And my trading partner is completely Windows based so until I
> convert him to Linux Windows must be part of the recipe.)
>
> Anyway, I try to keep my VMWare images below 10GB so that I can tar
> them up once a week and write them to a dual-layer DVD for backup.
> Once a month I move one to the safe deposit box at the bank. Otherwise
> I keep three in the house at the far end in a fireproof box. (Although
> high temp might damage them if the whole house goes up. Who knows...)
>
>   
Several thoughts about that, if you encrypt the tar you can burn another 
copy and stick it in with the CDs in your car. Which hopefully is not 
parked in an attached garage. And look at dvdisaster. It's a software 
ECC which gives you a much improved chance to recover data if you have 
issues with it.

Of course, if you have a trusted friend you can just do incremental over 
the net, unless you change all 10GB somehow. Makes for a slightly ugly 
recovery.

I finally went to Blu-Ray for full backup, DVD for incremental. I did do 
a "test once" and it worked fine to recover everything.

> So, Gentoo hosts VMWare. VMWare maps a logical 10GB Windows C: drive
> into a bunch of 2GB files and the stock data it typically a single
> file for each stock where the file size approaches 100MB. I have
> little or no control in terms of how that 100MB file is placed on the
> drive.
>
> I haven't a clue what that really means in terms of disk access from a
> Linux point of view.
>
> Daily backups are just my programming. They are incremental and just
> sent to another computer on the network, and then also covered in the
> VMWare backup at the end of the week.
>
>
> Replying to Asdo - I'm hoping to use a more or less standard desktop
> or server motherboard with 6-8 SATA ports dedicating 3-6 ports to this
> RAID interface and then using Linux software RAID and no hardware RAID
> controllers. Is this unreasonable in your opinion for the work flow
> I'm describing using VMWare?
>
> I'm concerned about any RAID hardware that leaves me stranded if the
> controller dies.
>   

In that case you start to look at server grade hardware, big UPS, or 
multiple systems doing realtime mirroring. If you want data center 
reliability you need data center paranoia and complexity. Decide what 
you need, and people will help you get there. When I ran the usenet 
servers for SBC we had a backup data center on another tectonic plate, 
think about how safe you want to be.

Other thought: have a backup off-site which captures a few days worth of 
new data, and do incrementals often. Worst case you have to recompute 
starting from a backup and whatever new data came in since.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein

Re: What RAID type and why?

[Michael Evans]

On Sat, Mar 6, 2010 at 3:17 PM, Guy Watkins <linux-raid@watkins-home.com> wrote:
> } -----Original Message-----
> } From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> } owner@vger.kernel.org] On Behalf Of Greg Freemyer
> } Sent: Saturday, March 06, 2010 5:33 PM
> } To: Mark Knecht
> } Cc: Linux-RAID
> } Subject: Re: What RAID type and why?
> }
> } On Sat, Mar 6, 2010 at 5:02 PM, Mark Knecht <markknecht@gmail.com> wrote:
> } > First post. I've never used RAID but am thinking about it and looking
> } > for newbie-level info. Thanks in advance.
> } >
> } > I'm thinking about building a machine for long term number crunching
> } > of stock market data. Highest end processor I can get, 16GB and at
> } > least reasonably fast drives. I've not done RAID before and don't know
> } > how to choose one RAID type over another for this sort of workload.
> } > All I know is I want the machine to run 24/7 computing 100% of the
> } > time and be reliable at least in the sense of not losing data if 1
> } > drive or possibly 2 go down.
> } >
> } > If a drive does go down I'm not overly worried about down time. I'll
> } > stock a couple of spares when I build the machine and power the box
> } > back up within an hour or two.
> } >
> } > What RAID type do I choose and why?
> } >
> } > Do I need a 5 physical drive RAID array to meet these requirements?
> } > Assume 1TB+ drives all around.
> } >
> } > How critical is it going forward with Linux RAID solutions to be able
> } > to get exactly the same drives in the future? 1TB today is 4TB a year
> } > from now, etc.
> } >
> } > With an 8 core processor (high-end Intel Core i7 probably) do I need
> } > to worry much about CPU usage doing RAID? I suspect not and I don't
> } > really want to get into hardware RAID controllers unless critically
> } > necessary which I suspect it isn't.
> } >
> } > Anyway, if there's a document around somewhere that helps a newbie
> } > like me I'd sure appreciate finding out about it.
> } >
> } > Thanks,
> } > Mark
> }
> } I'm not sure about a newbie doc, but here's some basics:
> }
> } You haven't said what kind of i/o rates you expect, nor how much
> } storage you need.
> }
> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> } which may be a problem.
>
> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
> and you don't have the RAID6 I/O overhead.
>
> Also, you said your data is important.  If so, you need a backup solution!
> 2 copies with 1 off-site.  Maybe alternate between the 2 each day or week.
>
> How much data per day?  How much data during the next 3 years?
>
> Guy
>
> }
> } Raid-5 is out of favor for me due to issues people are seeing with
> } discrete bad sectors with the remaining drives after you have a drive
> } failure.  raid-6 tolerates those much better.  Even raid 10 is not as
> } robust as raid 6 and with the current generation drives robustness in
> } the raid solution is more important than ever.
> }
> } But raid 6 uses 2 parity drives, so you'll only get 1TB of useable
> } space from a 3-disk raid 6 made from 1TB drives.
> }
> } mdraid just requires replacement disks be bigger than the old disk
> } you're replacing.
> }
> } You might consider layering LVM on top of mdraid to help you manage
> } the array as it grows.
> }
> } Greg
> } --
> } Greg Freemyer
> } Head of EDD Tape Extraction and Processing team
> } Litigation Triage Solutions Specialist
> } http://www.linkedin.com/in/gregfreemyer
> } Preservation and Forensic processing of Exchange Repositories White Paper
> } -
> } <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html>
> }
> } The Norcross Group
> } The Intersection of Evidence & Technology
> } http://www.norcrossgroup.com
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>

More importantly, it sounds like his workload will be mostly /database/ driven.

As far as I'm aware, databases tend to produce many small operations;
which unfortunately pushes favor to the simple mirroring operations.
If two drives going bad is a concern then using 2 backup copies per
raid 1 mirror set would work.  Most modern consumer systems come with
6 SATA ports or more, so it should be possible to get 6 hard drives
installed and shared among two raid 1 sets of 3 drives each.  LVM with
striping could be used over the raid 1 sets.

On the other hand, he says that the system will have 16 GB of memory;
I'm not sure what size his working set is, but it sounds entirely
plausible that a well constructed database could live entirely within
the ram.  If that's the case it doesn't really matter what the precise
performance of the storage solution is.  Raid 6 would offer more
efficient drive use at similar rates of error tolerance at a cost
savings of 2 drives in the six drive case.

Update for new email: Just go with the raid 1 version; it sounds like
you aren't trying to store terabytes of data so the raid 1 solution
with even just 3 drives should be sufficient.  Put the saved resources
in to more, faster, or better memory.
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Re: What RAID type and why?

[Neil Brown]

On Sat, 06 Mar 2010 18:17:44 -0500
"Guy Watkins" <linux-raid@watkins-home.com> wrote:

> } 
> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> } which may be a problem.
> 
> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
> and you don't have the RAID6 I/O overhead.
> 

and as md/raid6 requires at least 4 drives, RAID1 is not just the best
solution to survive two failures on a 3-device array, it is the only solution.

NeilBrown

Re: What RAID type and why?

[Keld Simonsen]

On Sun, Mar 07, 2010 at 01:21:13PM +1100, Neil Brown wrote:
> On Sat, 06 Mar 2010 18:17:44 -0500
> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
> 
> > } 
> > } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
> > } which may be a problem.
> > 
> > If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
> > and you don't have the RAID6 I/O overhead.
> > 
> 
> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
> solution to survive two failures on a 3-device array, it is the only solution.

Raid10 can also do it.

raid1 is in many ways obsolete and you should rather use raid10,
which in my eyeys is just another way of doing the same conceptual thing
as raid1.

Best regards
keld

RE: What RAID type and why?

[Guy Watkins]

} -----Original Message-----
} From: Keld Simonsen [mailto:keld@keldix.com]
} Sent: Sunday, March 07, 2010 3:07 AM
} To: Neil Brown
} Cc: Guy Watkins; 'Greg Freemyer'; 'Mark Knecht'; 'Linux-RAID'
} Subject: Re: What RAID type and why?
} 
} On Sun, Mar 07, 2010 at 01:21:13PM +1100, Neil Brown wrote:
} > On Sat, 06 Mar 2010 18:17:44 -0500
} > "Guy Watkins" <linux-raid@watkins-home.com> wrote:
} >
} > > }
} > > } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of
} i/o
} > > } which may be a problem.
} > >
} > > If he only needs 3 drives I would recommend RAID1.  Can still loose 2
} drives
} > > and you don't have the RAID6 I/O overhead.
} > >
} >
} > and as md/raid6 requires at least 4 drives, RAID1 is not just the best
} > solution to survive two failures on a 3-device array, it is the only
} solution.
} 
} Raid10 can also do it.
} 
} raid1 is in many ways obsolete and you should rather use raid10,
} which in my eyeys is just another way of doing the same conceptual thing
} as raid1.
} 
} Best regards
} keld

Are you sure RAID10 can loose 2 of 3 drives?  I did not think it worked that
way.  I thought RAID10 maintained 2 copies, not 3.  But I have never used
RAID10.

Guy

Re: What RAID type and why?

['Keld Simonsen']

On Sun, Mar 07, 2010 at 03:10:18AM -0500, Guy Watkins wrote:
> } -----Original Message-----
> } From: Keld Simonsen [mailto:keld@keldix.com]
> } Sent: Sunday, March 07, 2010 3:07 AM
> } To: Neil Brown
> } Cc: Guy Watkins; 'Greg Freemyer'; 'Mark Knecht'; 'Linux-RAID'
> } Subject: Re: What RAID type and why?
> } 
> } On Sun, Mar 07, 2010 at 01:21:13PM +1100, Neil Brown wrote:
> } > On Sat, 06 Mar 2010 18:17:44 -0500
> } > "Guy Watkins" <linux-raid@watkins-home.com> wrote:
> } >
> } > > }
> } > > } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of
> } i/o
> } > > } which may be a problem.
> } > >
> } > > If he only needs 3 drives I would recommend RAID1.  Can still loose 2
> } drives
> } > > and you don't have the RAID6 I/O overhead.
> } > >
> } >
> } > and as md/raid6 requires at least 4 drives, RAID1 is not just the best
> } > solution to survive two failures on a 3-device array, it is the only
> } solution.
> } 
> } Raid10 can also do it.
> } 
> } raid1 is in many ways obsolete and you should rather use raid10,
> } which in my eyeys is just another way of doing the same conceptual thing
> } as raid1.
> } 
> } Best regards
> } keld
> 
> Are you sure RAID10 can loose 2 of 3 drives?  I did not think it worked that
> way.  I thought RAID10 maintained 2 copies, not 3.  But I have never used
> RAID10.

If you ask mdadm to do it, yes. Example:

mdadm --create /dev/md3 --chunk=256 -R -l 10 -n 3 -p f3 /dev/sd[abc]1

the "-p f3" is the one that asks to have 3 copies.

best regards
keld

Re: What RAID type and why?

[Michael Evans]

On Sun, Mar 7, 2010 at 12:22 AM, Keld Simonsen <keld@keldix.com> wrote:
> On Sun, Mar 07, 2010 at 03:10:18AM -0500, Guy Watkins wrote:
>> } -----Original Message-----
>> } From: Keld Simonsen [mailto:keld@keldix.com]
>> } Sent: Sunday, March 07, 2010 3:07 AM
>> } To: Neil Brown
>> } Cc: Guy Watkins; 'Greg Freemyer'; 'Mark Knecht'; 'Linux-RAID'
>> } Subject: Re: What RAID type and why?
>> }
>> } On Sun, Mar 07, 2010 at 01:21:13PM +1100, Neil Brown wrote:
>> } > On Sat, 06 Mar 2010 18:17:44 -0500
>> } > "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>> } >
>> } > > }
>> } > > } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of
>> } i/o
>> } > > } which may be a problem.
>> } > >
>> } > > If he only needs 3 drives I would recommend RAID1.  Can still loose 2
>> } drives
>> } > > and you don't have the RAID6 I/O overhead.
>> } > >
>> } >
>> } > and as md/raid6 requires at least 4 drives, RAID1 is not just the best
>> } > solution to survive two failures on a 3-device array, it is the only
>> } solution.
>> }
>> } Raid10 can also do it.
>> }
>> } raid1 is in many ways obsolete and you should rather use raid10,
>> } which in my eyeys is just another way of doing the same conceptual thing
>> } as raid1.
>> }
>> } Best regards
>> } keld
>>
>> Are you sure RAID10 can loose 2 of 3 drives?  I did not think it worked that
>> way.  I thought RAID10 maintained 2 copies, not 3.  But I have never used
>> RAID10.
>
> If you ask mdadm to do it, yes. Example:
>
> mdadm --create /dev/md3 --chunk=256 -R -l 10 -n 3 -p f3 /dev/sd[abc]1
>
> the "-p f3" is the one that asks to have 3 copies.
>
> best regards
> keld
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>

Yes, that way would work, except in that case it would use more
complicated methods to split up the stripes among the drives.  Since
you're application seems to be read heavy, I agree with using 'far'
for the stripe method.

However the dis-advantage of mdadm raid10 has been two-fold compared
to raid1 (until kernel 2.6.33+).
1) Fixed in 2.6.33: Striped storage did not previously support
write-barriers (required for atomic write mechanisms/journals).
2) Still unsupported? : Reshape of raid10 arrays.
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Re: What RAID type and why?

[Goswin von Brederlow]

Neil Brown <neilb@suse.de> writes:

> On Sat, 06 Mar 2010 18:17:44 -0500
> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>
>> } 
>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>> } which may be a problem.
>> 
>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>> and you don't have the RAID6 I/O overhead.
>> 
>
> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
> solution to survive two failures on a 3-device array, it is the only solution.
>
> NeilBrown

Except that there also is raid10 with 3 mirrors. :)

MfG
        Goswin

PS: Why doesn't raid6 still not allow 3 drives for the special case of
converting raid1 -> raid6?

Re: What RAID type and why?

[Michael Evans]

On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Neil Brown <neilb@suse.de> writes:
>
>> On Sat, 06 Mar 2010 18:17:44 -0500
>> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>>
>>> }
>>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>>> } which may be a problem.
>>>
>>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>>> and you don't have the RAID6 I/O overhead.
>>>
>>
>> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
>> solution to survive two failures on a 3-device array, it is the only solution.
>>
>> NeilBrown
>
> Except that there also is raid10 with 3 mirrors. :)
>
> MfG
>        Goswin
>
> PS: Why doesn't raid6 still not allow 3 drives for the special case of
> converting raid1 -> raid6?
>
>
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> the body of a message to majordomo@vger.kernel.org
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>

That should be obvious:

Possible stripes:

Start:
1, 1, 1;
2, 2, 2;

'raid6' overtake...
1, q, Q;
2, q, Q;

'raid6' overtake with missing;
1, (missing 2), q, Q;
3, (missing 4), q, Q;

In the first overtake case you have the requirement of generating 200%
parity, which probably won't work for the algorithm and is a silly
idea in general since it's computationally far less expensive to store
another copy of either form of data instead.

In the second you're gaining the space of a second disk at the cost of
being already degraded; why not just go for raid 5 instead?

You can overtake raid5 later with raid6 if you add more devices.
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Re: What RAID type and why?

[Goswin von Brederlow]

Michael Evans <mjevans1983@gmail.com> writes:

> On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>> Neil Brown <neilb@suse.de> writes:
>>
>>> On Sat, 06 Mar 2010 18:17:44 -0500
>>> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>>>
>>>> }
>>>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>>>> } which may be a problem.
>>>>
>>>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>>>> and you don't have the RAID6 I/O overhead.
>>>>
>>>
>>> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
>>> solution to survive two failures on a 3-device array, it is the only solution.
>>>
>>> NeilBrown
>>
>> Except that there also is raid10 with 3 mirrors. :)
>>
>> MfG
>>        Goswin
>>
>> PS: Why doesn't raid6 still not allow 3 drives for the special case of
>> converting raid1 -> raid6?
>>
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
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>>
>
> That should be obvious:
>
> Possible stripes:
>
> Start:
> 1, 1, 1;
> 2, 2, 2;

Start:
1, 1, 1;
2, 2, 2;
3, 3, 3;
...

> 'raid6' overtake...
> 1, q, Q;
> 2, q, Q;

Middle:
1, P, Q;
P, Q, 2;
Q, 3, P;
...

End:
1, 2, P, Q;
4, P, Q, 3;
P, Q, 5, 6;
...

> 'raid6' overtake with missing;
> 1, (missing 2), q, Q;
> 3, (missing 4), q, Q;
>
> In the first overtake case you have the requirement of generating 200%
> parity, which probably won't work for the algorithm and is a silly
> idea in general since it's computationally far less expensive to store
> another copy of either form of data instead.

The sick 3 disk raid6 case should have both the P and Q identical to the
data block. It is indeed computational a waste to go through the
expensive P/Q parity algorithm for the same result as mirroring but this
is only ment as a transitional state.

> In the second you're gaining the space of a second disk at the cost of
> being already degraded; why not just go for raid 5 instead?
>
> You can overtake raid5 later with raid6 if you add more devices.

Because then you are going from 2 mirror disks to 1 parity disk even if
only temporary. You are reducing the number of disks failures you can
survive from 2 to 1 and the high load during a reshape makes a failure
more likely than normal operations.

Or can you go from 3 way raid1 to 4 disk raid6 in a single step?

MfG
        Goswin
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Re: What RAID type and why?

[Michael Evans]

On Wed, Mar 10, 2010 at 9:47 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Michael Evans <mjevans1983@gmail.com> writes:
>
>> On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>>> Neil Brown <neilb@suse.de> writes:
>>>
>>>> On Sat, 06 Mar 2010 18:17:44 -0500
>>>> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>>>>
>>>>> }
>>>>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>>>>> } which may be a problem.
>>>>>
>>>>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>>>>> and you don't have the RAID6 I/O overhead.
>>>>>
>>>>
>>>> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
>>>> solution to survive two failures on a 3-device array, it is the only solution.
>>>>
>>>> NeilBrown
>>>
>>> Except that there also is raid10 with 3 mirrors. :)
>>>
>>> MfG
>>>        Goswin
>>>
>>> PS: Why doesn't raid6 still not allow 3 drives for the special case of
>>> converting raid1 -> raid6?
>>>
>>>
>>> --
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>>> the body of a message to majordomo@vger.kernel.org
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>>>
>>
>> That should be obvious:
>>
>> Possible stripes:
>>
>> Start:
>> 1, 1, 1;
>> 2, 2, 2;
>
> Start:
> 1, 1, 1;
> 2, 2, 2;
> 3, 3, 3;
> ...
>
>> 'raid6' overtake...
>> 1, q, Q;
>> 2, q, Q;
>
> Middle:
> 1, P, Q;
> P, Q, 2;
> Q, 3, P;
> ...
>
> End:
> 1, 2, P, Q;
> 4, P, Q, 3;
> P, Q, 5, 6;
> ...
>
>> 'raid6' overtake with missing;
>> 1, (missing 2), q, Q;
>> 3, (missing 4), q, Q;
>>
>> In the first overtake case you have the requirement of generating 200%
>> parity, which probably won't work for the algorithm and is a silly
>> idea in general since it's computationally far less expensive to store
>> another copy of either form of data instead.
>
> The sick 3 disk raid6 case should have both the P and Q identical to the
> data block. It is indeed computational a waste to go through the
> expensive P/Q parity algorithm for the same result as mirroring but this
> is only ment as a transitional state.
>
>> In the second you're gaining the space of a second disk at the cost of
>> being already degraded; why not just go for raid 5 instead?
>>
>> You can overtake raid5 later with raid6 if you add more devices.
>
> Because then you are going from 2 mirror disks to 1 parity disk even if
> only temporary. You are reducing the number of disks failures you can
> survive from 2 to 1 and the high load during a reshape makes a failure
> more likely than normal operations.
>
> Or can you go from 3 way raid1 to 4 disk raid6 in a single step?
>
> MfG
>        Goswin
>

You are not planning on staying with 3 devices though.

Just stick with 2 redundancy raid 1 until you have four devices.
Then overtaking from raid 1 + hotspares at 4 devices total to raid 6
with 2 data devices and 2 parity devices per stripe makes sense.
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Re: What RAID type and why?

[Asdo]

Mark Knecht wrote:
> First post. I've never used RAID but am thinking about it and looking
> for newbie-level info. Thanks in advance.
>
> I'm thinking about building a machine for long term number crunching
> of stock market data. Highest end processor I can get, 16GB and at
> least reasonably fast drives. I've not done RAID before and don't know
> how to choose one RAID type over another for this sort of workload.
> All I know is I want the machine to run 24/7 computing 100% of the
> time and be reliable at least in the sense of not losing data if 1
> drive or possibly 2 go down.
>
> If a drive does go down I'm not overly worried about down time. I'll
> stock a couple of spares when I build the machine and power the box
> back up within an hour or two.
>
> What RAID type do I choose and why?
>
> Do I need a 5 physical drive RAID array to meet these requirements?
> Assume 1TB+ drives all around.
>
> How critical is it going forward with Linux RAID solutions to be able
> to get exactly the same drives in the future? 1TB today is 4TB a year
> from now, etc.
>   
Hi Mark

I'll reply to just a few points.

You don't need the same drives, only a few requirements:

1 - The drives need to play well with the controller. Do some tests. 
There were rare cases of certain drives being dropped by certain 
controllers e.g. on high I/O. Just do some tests before putting valuable 
data in. Maybe look at the HCL list for your controller before buying.

2 - the new drives need to be at least as large as the old ones. Extra 
space will be wasted (ok not exactly, you can use the extra space for 
other purposes). Speed is not relevant for bare functionality.

3 - It's better if new drives are not slower than the older drives. 
Everything moves at the speed of the slowest drive.

4 - Better to take raid-edition or enterprise-grade drives, especially 
because they usually have RTL "recovery time limit" also called TLER 
(time limited error recovery). Go google for it to understand why it is 
useful in RAID.

> With an 8 core processor (high-end Intel Core i7 probably) do I need
> to worry much about CPU usage doing RAID? I suspect not and I don't
> really want to get into hardware RAID controllers unless critically
> necessary which I suspect it isn't.
>   

I think for a 5 disks raid array the CPU power is still not the limiting 
factor. Especially not in the fast raids like raid10. Note that only 1 
core will be used for RAID parity computation for raid456. All cores 
will be used for handling interrupts from the drives though.

Difficult to suggest what RAID you should use. We don't know access 
patterns, speed requirements, value of your data.

Minimum for redundancy is raid-1 on 2 drives.