Building RAID5 w/o a Hot Spare

Building RAID5 w/o a Hot Spare

[Carlos Williams]

I am building a Linux system which has 4 identical 'fd' partitions on
each of the 4 drives. I don't want to use the 4th partition or disk if
you would like to call it as a 'hot spare'. I would like to span the
entire array over the 4 drives. My question is does my command look
correct for doing so?

mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda2 /dev/sdb2
/dev/sdc2 /dev/sdd2

I shouldn't have to specify any additional flags in the 'mdadm'
command to make sure it uses all 4 drives as the RAID5 array, right?

Re: Building RAID5 w/o a Hot Spare

[Michał Sawicz]

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Dnia 2010-01-27, śro o godzinie 09:26 -0500, Carlos Williams pisze:
> I am building a Linux system which has 4 identical 'fd' partitions on
> each of the 4 drives. I don't want to use the 4th partition or disk if
> you would like to call it as a 'hot spare'. I would like to span the
> entire array over the 4 drives. My question is does my command look
> correct for doing so?

I'm not sure I understand what you want to achieve. A 'spare' disk is
one that isn't used until any of the other fail, then the spare replaces
the failed one. The term 'hot spare' is used to identify a disk that is
a mirror of one of the other drives, but there's no support for that in
Linux Software-RAID.

> mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda2 /dev/sdb2
> /dev/sdc2 /dev/sdd2 

This will create a RAID5 array of 4 devices, yes. Capacity will be 3 x
sd?2. Failure of any one of those disks will not harm the data.

-- 
Cheers
Michał (Saviq) Sawicz


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Re: Building RAID5 w/o a Hot Spare

[Robin Hill]

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On Wed Jan 27, 2010 at 03:46:17PM +0100, Michał Sawicz wrote:

> Dnia 2010-01-27, śro o godzinie 09:26 -0500, Carlos Williams pisze:
> > I am building a Linux system which has 4 identical 'fd' partitions on
> > each of the 4 drives. I don't want to use the 4th partition or disk if
> > you would like to call it as a 'hot spare'. I would like to span the
> > entire array over the 4 drives. My question is does my command look
> > correct for doing so?
> 
> I'm not sure I understand what you want to achieve. A 'spare' disk is
> one that isn't used until any of the other fail, then the spare replaces
> the failed one. The term 'hot spare' is used to identify a disk that is
> a mirror of one of the other drives, but there's no support for that in
> Linux Software-RAID.
> 
A hot spare is one that's plugged in, powered up, and available for use
(a cold spare is sat on a shelf).  At least, that's the only way I've
ever heard the terminology used.

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

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Re: Building RAID5 w/o a Hot Spare

[Bill Davidsen]

Michał Sawicz wrote:
> Dnia 2010-01-27, śro o godzinie 09:26 -0500, Carlos Williams pisze:
>   
>> I am building a Linux system which has 4 identical 'fd' partitions on
>> each of the 4 drives. I don't want to use the 4th partition or disk if
>> you would like to call it as a 'hot spare'. I would like to span the
>> entire array over the 4 drives. My question is does my command look
>> correct for doing so?
>>     
>
> I'm not sure I understand what you want to achieve. A 'spare' disk is
> one that isn't used until any of the other fail, then the spare replaces
> the failed one. The term 'hot spare' is used to identify a disk that is
> a mirror of one of the other drives, but there's no support for that in
> Linux Software-RAID.
>
>   
>> mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda2 /dev/sdb2
>> /dev/sdc2 /dev/sdd2 
>>     
>
> This will create a RAID5 array of 4 devices, yes. Capacity will be 3 x
> sd?2. Failure of any one of those disks will not harm the data.
>
>   
A hot spare is in and spinning, or at least powered up, ready to use 
with software. And software raid does support putting a raid, such as 
raid-1 or even raid-5, or members which are raid-1 with a write mostly 
drive and ready to go.

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


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Re: Building RAID5 w/o a Hot Spare

[Goswin von Brederlow]

Carlos Williams <carloswill@gmail.com> writes:

> I am building a Linux system which has 4 identical 'fd' partitions on
> each of the 4 drives. I don't want to use the 4th partition or disk if
> you would like to call it as a 'hot spare'. I would like to span the
> entire array over the 4 drives. My question is does my command look
> correct for doing so?
>
> mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda2 /dev/sdb2
> /dev/sdc2 /dev/sdd2
>
> I shouldn't have to specify any additional flags in the 'mdadm'
> command to make sure it uses all 4 drives as the RAID5 array, right?

No, it will use 4 drives as specified. Linux mdadm doesn't get ideas on
its own. It does what you tell it to do. :)

MfG
        Goswin

PS: sda2 is usualy not the 4th partition on a drive

Re: Building RAID5 w/o a Hot Spare

[Carlos Williams]

On Wed, Jan 27, 2010 at 10:14 AM, Goswin von Brederlow
<goswin-v-b@web.de> wrote:
> PS: sda2 is usualy not the 4th partition on a drive

Why would I have thought it was? 'sda"2" would be the 2nd partition on a drive.

Re: Building RAID5 w/o a Hot Spare

[Goswin von Brederlow]

Carlos Williams <carloswill@gmail.com> writes:

> On Wed, Jan 27, 2010 at 10:14 AM, Goswin von Brederlow
> <goswin-v-b@web.de> wrote:
>> PS: sda2 is usualy not the 4th partition on a drive
>
> Why would I have thought it was? 'sda"2" would be the 2nd partition on a drive.
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> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Sorry, I was just confused by your use of "4th partition" in your mail
but you ment disk there. NM

MfG
        Goswin

Re: Building RAID5 w/o a Hot Spare

[Carlos Williams]

2010/1/27 Michał Sawicz <michal@sawicz.net>:
> I'm not sure I understand what you want to achieve. A 'spare' disk is
> one that isn't used until any of the other fail, then the spare replaces
> the failed one. The term 'hot spare' is used to identify a disk that is
> a mirror of one of the other drives, but there's no support for that in
> Linux Software-RAID.

I am sorry because I think I was using the wrong terminology then. I
want to use all 4 disks to be an active member or the RAID5 /dev/md0.
I wanted to make sure the command I posted in my original post was not
going to use 3 partitions for active members of RAID5 on /dev/md0 and
use the 4th drive as a 'spare device' in case one of the three drives
fail. I don't want that. I just want 4 active / working devices for
/dev/md0.

>> mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda2 /dev/sdb2
>> /dev/sdc2 /dev/sdd2
>
> This will create a RAID5 array of 4 devices, yes. Capacity will be 3 x
> sd?2. Failure of any one of those disks will not harm the data.

OK. I think that answers my questions. Thanks for your help!
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Re: Building RAID5 w/o a Hot Spare

[Michał Sawicz]

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Dnia 2010-01-27, śro o godzinie 09:41 -0500, Carlos Williams pisze:
> 2010/1/27 Michał Sawicz <michal@sawicz.net>:
> > I'm not sure I understand what you want to achieve. A 'spare' disk
> is
> > one that isn't used until any of the other fail, then the spare
> replaces
> > the failed one. The term 'hot spare' is used to identify a disk that
> is
> > a mirror of one of the other drives, but there's no support for that
> in
> > Linux Software-RAID.
> 
> I am sorry because I think I was using the wrong terminology then. I
> want to use all 4 disks to be an active member or the RAID5 /dev/md0.
> I wanted to make sure the command I posted in my original post was not
> going to use 3 partitions for active members of RAID5 on /dev/md0 and
> use the 4th drive as a 'spare device' in case one of the three drives
> fail. I don't want that. I just want 4 active / working devices for
> /dev/md0.
> 
> >> mdadm --create /dev/md0 --level=5
> --raid-devices=4 /dev/sda2 /dev/sdb2
> >> /dev/sdc2 /dev/sdd2
> >
> > This will create a RAID5 array of 4 devices, yes. Capacity will be 3
> x
> > sd?2. Failure of any one of those disks will not harm the data.
> 
> OK. I think that answers my questions. Thanks for your help!

Great.

Please keep the mailing list in the loop.

-- 
Cheers
Michał (Saviq) Sawicz

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