3 disk RAID1?

3 disk RAID1?

[Mark Knecht]

Hi all,
   I'm still very much on a steep learning curve about what I can do
with Linux software RAID. In another thread this weekend a couple of
responders discussed among themselves 3-disk RAID1 solutions that can
survive if 2 disks die. I don't understand what that means. Can
someone point me at a quick explanation? Is that really possible?

   In general I'm using a few Wikipedia pages and gravitate toward the
diagrams as much as anything.

http://en.wikipedia.org/wiki/RAID
http://en.wikipedia.org/wiki/RAID1#RAID_1

RAID0 - striping, speed not reliability (2 disk minimum)
RAID1 - duplicate data, no other protection (2 disk minimum)

   How do I build RAID1 using three drives? Just duplicate the data 3
times? If drives start going bad how do I determine which one or two
are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
straightforward to figure out, but 2?

Thanks,
Mark

Re: 3 disk RAID1?

[Robin Hill]

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On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:

> Hi all,
>    I'm still very much on a steep learning curve about what I can do
> with Linux software RAID. In another thread this weekend a couple of
> responders discussed among themselves 3-disk RAID1 solutions that can
> survive if 2 disks die. I don't understand what that means. Can
> someone point me at a quick explanation? Is that really possible?
> 
>    In general I'm using a few Wikipedia pages and gravitate toward the
> diagrams as much as anything.
> 
> http://en.wikipedia.org/wiki/RAID
> http://en.wikipedia.org/wiki/RAID1#RAID_1
> 
> RAID0 - striping, speed not reliability (2 disk minimum)
> RAID1 - duplicate data, no other protection (2 disk minimum)
> 
>    How do I build RAID1 using three drives? Just duplicate the data 3
> times? If drives start going bad how do I determine which one or two
> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
> straightforward to figure out, but 2?
> 
A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
against hardware failures, so you know which disk has failed because it
gets kicked out of the array as faulty.  This is the same regardless of
how many mirrored copies you have (md will detect a write failure to a
drive and mark it as faulty - read errors will cause the failed block to
get rewritten).

As for how to create it - it's just the same process as for a 2-disk
RAID1 but specifying 3 drives (assuming you're using Linux md software
RAID - if not, please specify what you're intending to use).  The manual
page for mdadm should give you everything you need - do ask if there's
anything you want clarifying though.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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Re: 3 disk RAID1?

[Mark Knecht]

On Mon, Mar 8, 2010 at 12:58 PM, Robin Hill <robin@robinhill.me.uk> wrote:
> On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:
>
>> Hi all,
>>    I'm still very much on a steep learning curve about what I can do
>> with Linux software RAID. In another thread this weekend a couple of
>> responders discussed among themselves 3-disk RAID1 solutions that can
>> survive if 2 disks die. I don't understand what that means. Can
>> someone point me at a quick explanation? Is that really possible?
>>
>>    In general I'm using a few Wikipedia pages and gravitate toward the
>> diagrams as much as anything.
>>
>> http://en.wikipedia.org/wiki/RAID
>> http://en.wikipedia.org/wiki/RAID1#RAID_1
>>
>> RAID0 - striping, speed not reliability (2 disk minimum)
>> RAID1 - duplicate data, no other protection (2 disk minimum)
>>
>>    How do I build RAID1 using three drives? Just duplicate the data 3
>> times? If drives start going bad how do I determine which one or two
>> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
>> straightforward to figure out, but 2?
>>
> A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
> against hardware failures, so you know which disk has failed because it
> gets kicked out of the array as faulty.  This is the same regardless of
> how many mirrored copies you have (md will detect a write failure to a
> drive and mark it as faulty - read errors will cause the failed block to
> get rewritten).
>
> As for how to create it - it's just the same process as for a 2-disk
> RAID1 but specifying 3 drives (assuming you're using Linux md software
> RAID - if not, please specify what you're intending to use).  The manual
> page for mdadm should give you everything you need - do ask if there's
> anything you want clarifying though.
>
> Cheers,
>    Robin

Thanks Robin. Maybe I am getting smarter about this if I'm figuring
out what others are talking about! ;-)

Cheers,
Mark
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Re: 3 disk RAID1?

[Michael Evans]

On Mon, Mar 8, 2010 at 1:04 PM, Mark Knecht <markknecht@gmail.com> wrote:
> On Mon, Mar 8, 2010 at 12:58 PM, Robin Hill <robin@robinhill.me.uk> wrote:
>> On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:
>>
>>> Hi all,
>>>    I'm still very much on a steep learning curve about what I can do
>>> with Linux software RAID. In another thread this weekend a couple of
>>> responders discussed among themselves 3-disk RAID1 solutions that can
>>> survive if 2 disks die. I don't understand what that means. Can
>>> someone point me at a quick explanation? Is that really possible?
>>>
>>>    In general I'm using a few Wikipedia pages and gravitate toward the
>>> diagrams as much as anything.
>>>
>>> http://en.wikipedia.org/wiki/RAID
>>> http://en.wikipedia.org/wiki/RAID1#RAID_1
>>>
>>> RAID0 - striping, speed not reliability (2 disk minimum)
>>> RAID1 - duplicate data, no other protection (2 disk minimum)
>>>
>>>    How do I build RAID1 using three drives? Just duplicate the data 3
>>> times? If drives start going bad how do I determine which one or two
>>> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
>>> straightforward to figure out, but 2?
>>>
>> A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
>> against hardware failures, so you know which disk has failed because it
>> gets kicked out of the array as faulty.  This is the same regardless of
>> how many mirrored copies you have (md will detect a write failure to a
>> drive and mark it as faulty - read errors will cause the failed block to
>> get rewritten).
>>
>> As for how to create it - it's just the same process as for a 2-disk
>> RAID1 but specifying 3 drives (assuming you're using Linux md software
>> RAID - if not, please specify what you're intending to use).  The manual
>> page for mdadm should give you everything you need - do ask if there's
>> anything you want clarifying though.
>>
>> Cheers,
>>    Robin
>
> Thanks Robin. Maybe I am getting smarter about this if I'm figuring
> out what others are talking about! ;-)
>
> Cheers,
> Mark
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> 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
>

When in doubt, read the manual two or three more times.

This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
some background comparison sections when I made that...
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Re: 3 disk RAID1?

[Kristleifur Daðason]

On Mon, Mar 8, 2010 at 10:11 PM, Michael Evans <mjevans1983@gmail.com> wrote:
> On Mon, Mar 8, 2010 at 1:04 PM, Mark Knecht <markknecht@gmail.com> wrote:
>> On Mon, Mar 8, 2010 at 12:58 PM, Robin Hill <robin@robinhill.me.uk> wrote:
>>> On Mon Mar 08, 2010 at 12:39:26PM -0800, Mark Knecht wrote:
>>>
>>>> Hi all,
>>>>    I'm still very much on a steep learning curve about what I can do
>>>> with Linux software RAID. In another thread this weekend a couple of
>>>> responders discussed among themselves 3-disk RAID1 solutions that can
>>>> survive if 2 disks die. I don't understand what that means. Can
>>>> someone point me at a quick explanation? Is that really possible?
>>>>
>>>>    In general I'm using a few Wikipedia pages and gravitate toward the
>>>> diagrams as much as anything.
>>>>
>>>> http://en.wikipedia.org/wiki/RAID
>>>> http://en.wikipedia.org/wiki/RAID1#RAID_1
>>>>
>>>> RAID0 - striping, speed not reliability (2 disk minimum)
>>>> RAID1 - duplicate data, no other protection (2 disk minimum)
>>>>
>>>>    How do I build RAID1 using three drives? Just duplicate the data 3
>>>> times? If drives start going bad how do I determine which one or two
>>>> are failing? (fsck? SMART?) With 3 drives 1 fail seems relatively
>>>> straightforward to figure out, but 2?
>>>>
>>> A 3-disk RAID1 is just 3 duplicate copies, yes.  And RAID only protects
>>> against hardware failures, so you know which disk has failed because it
>>> gets kicked out of the array as faulty.  This is the same regardless of
>>> how many mirrored copies you have (md will detect a write failure to a
>>> drive and mark it as faulty - read errors will cause the failed block to
>>> get rewritten).
>>>
>>> As for how to create it - it's just the same process as for a 2-disk
>>> RAID1 but specifying 3 drives (assuming you're using Linux md software
>>> RAID - if not, please specify what you're intending to use).  The manual
>>> page for mdadm should give you everything you need - do ask if there's
>>> anything you want clarifying though.
>>>
>>> Cheers,
>>>    Robin
>>
>> Thanks Robin. Maybe I am getting smarter about this if I'm figuring
>> out what others are talking about! ;-)
>>
>> 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
>>
>
> When in doubt, read the manual two or three more times.
>
> This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
> some background comparison sections when I made that...

Good advice.

The contents of the manual didn't really stick for me until I'd
actually *done* the tasks I was trying to figure out. The beautiful
and classic Catch-22 of learning new things. So I somewhat
empathically suggest you obtain a virtual machine program and start up
a few disposable virtual machines. (VirtualBox is pretty good and also
free.) Try creating some RAID's on VM's and getting them to boot, try
failing a device and readding it, etc.

And keep the manual in a window next to the virtual machine's window :)

-- Kristleifur
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Re: 3 disk RAID1?

[Mark Knecht]

On Mon, Mar 8, 2010 at 2:18 PM, Kristleifur Daðason
<kristleifur@gmail.com> wrote:
<SNIP>
>>
>> When in doubt, read the manual two or three more times.
>>
>> This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
>> some background comparison sections when I made that...
>
> Good advice.
>
> The contents of the manual didn't really stick for me until I'd
> actually *done* the tasks I was trying to figure out. The beautiful
> and classic Catch-22 of learning new things. So I somewhat
> empathically suggest you obtain a virtual machine program and start up
> a few disposable virtual machines. (VirtualBox is pretty good and also
> free.) Try creating some RAID's on VM's and getting them to boot, try
> failing a device and readding it, etc.
>
> And keep the manual in a window next to the virtual machine's window :)
>
> -- Kristleifur
>

Exactly what I'm working on this afternoon. I've not created many VM's
from scratch so there's a learning curve there, but potentially making
the hard drives virtual in the new VM setup and then applying RAID to
them seems like a good way to practice.

Additionally I'm going to purchase maybe 3 or 4 drives and then see
about using the other SATA ports on my desktop motherboard to create
additional storage that I can get some real performance numbers on.

Thanks!

- Mark
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RE: 3 disk RAID1?

[Leslie Rhorer]

> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> owner@vger.kernel.org] On Behalf Of Mark Knecht
> Sent: Monday, March 08, 2010 7:19 PM
> To: Kristleifur Daðason
> Cc: Linux-RAID
> Subject: Re: 3 disk RAID1?
> 
> On Mon, Mar 8, 2010 at 2:18 PM, Kristleifur Daðason
> <kristleifur@gmail.com> wrote:
> <SNIP>
> >>
> >> When in doubt, read the manual two or three more times.
> >>
> >> This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
> >> some background comparison sections when I made that...
> >
> > Good advice.
> >
> > The contents of the manual didn't really stick for me until I'd
> > actually *done* the tasks I was trying to figure out. The beautiful
> > and classic Catch-22 of learning new things. So I somewhat
> > empathically suggest you obtain a virtual machine program and start up
> > a few disposable virtual machines. (VirtualBox is pretty good and also
> > free.) Try creating some RAID's on VM's and getting them to boot, try
> > failing a device and readding it, etc.
> >
> > And keep the manual in a window next to the virtual machine's window :)
> >
> > -- Kristleifur
> >
> 
> Exactly what I'm working on this afternoon. I've not created many VM's
> from scratch so there's a learning curve there, but potentially making
> the hard drives virtual in the new VM setup and then applying RAID to
> them seems like a good way to practice.
> 
> Additionally I'm going to purchase maybe 3 or 4 drives and then see
> about using the other SATA ports on my desktop motherboard to create
> additional storage that I can get some real performance numbers on.

	Your mileage may definitely vary, but neither of the servers on
which I run mdadm have motherboards which allow mdadm to recognize the
drives.  For both of them, only drives 0 and 1 are properly recognized by
Linux.  I had to purchase SATA controllers for both.  Both are Asus AMD-64
motherboards.

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Re: 3 disk RAID1?

[Michael Evans]

Quite odd, my AMD64 asus mobos work just fine.  Of course I have been
avoiding all NForce chipsets for years.

Maybe you should set the BIOS so that the discs are presented as AHCI devices?

On Mon, Mar 8, 2010 at 5:26 PM, Leslie Rhorer <lrhorer@satx.rr.com> wrote:
>
>
>> -----Original Message-----
>> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
>> owner@vger.kernel.org] On Behalf Of Mark Knecht
>> Sent: Monday, March 08, 2010 7:19 PM
>> To: Kristleifur Dašason
>> Cc: Linux-RAID
>> Subject: Re: 3 disk RAID1?
>>
>> On Mon, Mar 8, 2010 at 2:18 PM, Kristleifur Dašason
>> <kristleifur@gmail.com> wrote:
>> <SNIP>
>> >>
>> >> When in doubt, read the manual two or three more times.
>> >>
>> >> This might also help you: http://wiki.tldp.org/LVM-on-RAID  I wrote
>> >> some background comparison sections when I made that...
>> >
>> > Good advice.
>> >
>> > The contents of the manual didn't really stick for me until I'd
>> > actually *done* the tasks I was trying to figure out. The beautiful
>> > and classic Catch-22 of learning new things. So I somewhat
>> > empathically suggest you obtain a virtual machine program and start up
>> > a few disposable virtual machines. (VirtualBox is pretty good and also
>> > free.) Try creating some RAID's on VM's and getting them to boot, try
>> > failing a device and readding it, etc.
>> >
>> > And keep the manual in a window next to the virtual machine's window :)
>> >
>> > -- Kristleifur
>> >
>>
>> Exactly what I'm working on this afternoon. I've not created many VM's
>> from scratch so there's a learning curve there, but potentially making
>> the hard drives virtual in the new VM setup and then applying RAID to
>> them seems like a good way to practice.
>>
>> Additionally I'm going to purchase maybe 3 or 4 drives and then see
>> about using the other SATA ports on my desktop motherboard to create
>> additional storage that I can get some real performance numbers on.
>
>        Your mileage may definitely vary, but neither of the servers on
> which I run mdadm have motherboards which allow mdadm to recognize the
> drives.  For both of them, only drives 0 and 1 are properly recognized by
> Linux.  I had to purchase SATA controllers for both.  Both are Asus AMD-64
> motherboards.
>
> --
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> 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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