raid10 How do I tell what side of the mirror a disk is on?

raid10 How do I tell what side of the mirror a disk is on?

[Rick Bragg]

Hi,

I recently posted a question to find out how to tell what is sda, b, c,
etc... and I am now using smartctl to get the serial numbers and all is
really great.  Thanks everyone for that info!

Another puzzle is with raid10, there are 2 used in a stripe, then that
stripe is mirrored with the other 2.  My question is how can I tell
"what 2 disks can fail?"  In other words, how can I tell for example if
sda and b are stiped? and that c and d are a mirror of a and b?  How can
I see what is happening with that?  Is there a way to tell?  

Thanks
Rick

Re: raid10 How do I tell what side of the mirror a disk is on?

[Goswin von Brederlow]

Rick Bragg <lists@gmnet.net> writes:

> Hi,
>
> I recently posted a question to find out how to tell what is sda, b, c,
> etc... and I am now using smartctl to get the serial numbers and all is
> really great.  Thanks everyone for that info!
>
> Another puzzle is with raid10, there are 2 used in a stripe, then that
> stripe is mirrored with the other 2.  My question is how can I tell
> "what 2 disks can fail?"  In other words, how can I tell for example if
> sda and b are stiped? and that c and d are a mirror of a and b?  How can
> I see what is happening with that?  Is there a way to tell?  
>
> Thanks
> Rick

Does it matter? If one disk fails you know the next disk failure has a
33+% chance of data loss and knowing which disk will be critical doesn't
change that.

MfG
        Goswin

PS: why not set up a raid over /dev/ramX with the same option of your
real raid, fail some disks and see when it breaks down.

Re: raid10 How do I tell what side of the mirror a disk is on?

[Michał Sawicz]

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Dnia 2010-01-30, sob o godzinie 00:50 +0100, Goswin von Brederlow pisze:
> Does it matter? If one disk fails you know the next disk failure has a
> 33+% chance of data loss and knowing which disk will be critical
> doesn't
> change that. 

For me it was important when I needed to have the array running (root
fs) with only 2 disks out of 4 (not enough controllers). It would be
nice to be able to tell which are mirrors, which are striped.

-- 
Cheers
Michał (Saviq) Sawicz

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Re: raid10 How do I tell what side of the mirror a disk is on?

[Michael Evans]

2010/1/29 Michał Sawicz <michal@sawicz.net>:
> Dnia 2010-01-30, sob o godzinie 00:50 +0100, Goswin von Brederlow pisze:
>> Does it matter? If one disk fails you know the next disk failure has a
>> 33+% chance of data loss and knowing which disk will be critical
>> doesn't
>> change that.
>
> For me it was important when I needed to have the array running (root
> fs) with only 2 disks out of 4 (not enough controllers). It would be
> nice to be able to tell which are mirrors, which are striped.
>
> --
> Cheers
> Michał (Saviq) Sawicz
>

As was noted in another reply recently; write down drive serial
numbers.  Drive serial numbers can be obtained online via tools like
smartctl or hdparm (read their manuals, they're useful tools).

In your case you have a 50% chance of finding a working combination
the first time, and swapping only one cable from a disk that is
connected to one that's not would resolve the issue.
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Re: raid10 How do I tell what side of the mirror a disk is on?

[Michał Sawicz]

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Dnia 2010-01-29, pią o godzinie 23:22 -0800, Michael Evans pisze:
> As was noted in another reply recently; write down drive serial
> numbers.  Drive serial numbers can be obtained online via tools like
> smartctl or hdparm (read their manuals, they're useful tools).

That I knew already, and I knew which disks are which, but there was no
way to tell which of them are mirrored, which are striped.

> In your case you have a 50% chance of finding a working combination
> the first time, and swapping only one cable from a disk that is
> connected to one that's not would resolve the issue. 

Yes, of course - I even got it the first time, but still - it would be
nice to be sure from /proc/mdstat or mdadm -E.

-- 
Cheers
Michał (Saviq) Sawicz

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Re: raid10 How do I tell what side of the mirror a disk is on?

[Robin Hill]

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On Fri Jan 29, 2010 at 10:00:47AM -0500, Rick Bragg wrote:

> Another puzzle is with raid10, there are 2 used in a stripe, then that
> stripe is mirrored with the other 2.  My question is how can I tell
> "what 2 disks can fail?"  In other words, how can I tell for example if
> sda and b are stiped? and that c and d are a mirror of a and b?  How can
> I see what is happening with that?  Is there a way to tell?  
> 
Reading the RAID10 documentation (man md, or check Wikipedia), with
four disks the adjacent disks should always contain the same data.  So
for sda, sdb, sdc, sdd (in that order in the array), you can lose one of
sda/sdb and one of sdc/sdd.  This holds for any layout (near, far or
offset) with the default 2 copies.

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

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