RAID1: What happens when one half of a mirror fails?

RAID1: What happens when one half of a mirror fails?

[Eric Pretorious]

Hello, All:

I've been a bit surprised by the complete failure of our system when one half 
of a mirror failed so I suppose that I should verify that I understand 
RAID1's failover methodology: Is the purpose of the mirror only to preserve 
the data or is it to allow the system to continue operating until a 
replacement can be installed? (i.e., If one half of a mirror fails, shouldn't 
the system continue operating normally?)

-- 
Eric P.,
Truckee, CA

Re: RAID1: What happens when one half of a mirror fails?

[Frank Wittig]

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The RAID1 should allow the system to opperate as usual until the drive
is replaced. I replaced 2 hot-plugable disks (both part two RAID1 which
were both parts of a RAID0) on a dell poweredge server this tuestay
without even rebooting.
If your disks have EEC there should be a minimum risk for failure of a
degraded RAID1.

Eric Pretorious wrote:
> Hello, All:
> 
> I've been a bit surprised by the complete failure of our system when one half 
> of a mirror failed so I suppose that I should verify that I understand 
> RAID1's failover methodology: Is the purpose of the mirror only to preserve 
> the data or is it to allow the system to continue operating until a 
> replacement can be installed? (i.e., If one half of a mirror fails, shouldn't 
> the system continue operating normally?)
> 


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Re: RAID1: What happens when one half of a mirror fails?

[Laurent CARON]

Eric Pretorious a écrit :

>Hello, All:
>
>I've been a bit surprised by the complete failure of our system when one half 
>of a mirror failed so I suppose that I should verify that I understand 
>RAID1's failover methodology: Is the purpose of the mirror only to preserve 
>the data or is it to allow the system to continue operating until a 
>replacement can be installed? (i.e., If one half of a mirror fails, shouldn't 
>the system continue operating normally?)
>
>  
>
is the swap partition on raid?

-- 
Mauvaise garde permet au loup de se repaître.
	-+- Le Roman de Renart -+-

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Re: RAID1: What happens when one half of a mirror fails?

[Eric Pretorious]

On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
>is the swap partition on raid?

No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.

-- 
Eric P.,
Truckee, CA

Re: RAID1: What happens when one half of a mirror fails?

[Frank Wittig]

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Eric Pretorious wrote:
> On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
> 
>>is the swap partition on raid?
> 
> 
> No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.
> 

That's no good idea. If a disk fails which contains a swap partition to
which data has been swapped the system will crash.
Therefor it is important to use mirrored partitions for swap.

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Re: RAID1: What happens when one half of a mirror fails?

[raz ben jehuda]

i had a "system hanging" when both swap and root file system were on the
raid. for that i added line in the kernel that removes the bad disk from
the operating system. 
On Thu, 2005-06-30 at 22:53, Frank Wittig wrote:
> Eric Pretorious wrote:
> > On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
> > 
> >>is the swap partition on raid?
> > 
> > 
> > No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.
> > 
> 
> That's no good idea. If a disk fails which contains a swap partition to
> which data has been swapped the system will crash.
> Therefor it is important to use mirrored partitions for swap.
-- 
Raz
Long Live The Penguin

Re: RAID1: What happens when one half of a mirror fails?

[Frank Wittig]

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with current version 2.6 kernels there are no problems known to me not
to have swap and system raid partitions built out of partitions of the
same physical disks.
i didn't catch what you mean by "the kernel that removes the bad disk
from the operating system". if it means that it will trigger a swapoff
for the partition on the failed disk it is near to useless since at that
time already a irreversible data loss has happened which causes the
system to crash.
the only possibility to prevent a system crash during failure of a
harddisk is swapping to raid partitions.

raz ben jehuda wrote:
> i had a "system hanging" when both swap and root file system were on the
> raid. for that i added line in the kernel that removes the bad disk from
> the operating system. 
> On Thu, 2005-06-30 at 22:53, Frank Wittig wrote:
> 
>>Eric Pretorious wrote:
>>
>>>On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
>>>
>>>
>>>>is the swap partition on raid?
>>>
>>>
>>>No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.
>>>
>>
>>That's no good idea. If a disk fails which contains a swap partition to
>>which data has been swapped the system will crash.
>>Therefor it is important to use mirrored partitions for swap.


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Re: RAID1: What happens when one half of a mirror fails?

[raz ben jehuda]

it means that your system will not crash. no data loss would happen 
The problem i had was that the system kept on bouncing io to the
defected disk and since both the root file system and the swap were on 
raid1 the system hanged after a short while.
I added a line that removed the defected ata port ( the disk ) from the
disks array in the kernel and by doing that i had prevented this io
bouncing.  


On Sun, 2005-07-03 at 18:43, Frank Wittig wrote:
> with current version 2.6 kernels there are no problems known to me not
> to have swap and system raid partitions built out of partitions of the
> same physical disks.
> i didn't catch what you mean by "the kernel that removes the bad disk
> from the operating system". if it means that it will trigger a swapoff
> for the partition on the failed disk it is near to useless since at that
> time already a irreversible data loss has happened which causes the
> system to crash.
> the only possibility to prevent a system crash during failure of a
> harddisk is swapping to raid partitions.
> 
> raz ben jehuda wrote:
> > i had a "system hanging" when both swap and root file system were on the
> > raid. for that i added line in the kernel that removes the bad disk from
> > the operating system. 
> > On Thu, 2005-06-30 at 22:53, Frank Wittig wrote:
> > 
> >>Eric Pretorious wrote:
> >>
> >>>On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
> >>>
> >>>
> >>>>is the swap partition on raid?
> >>>
> >>>
> >>>No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.
> >>>
> >>
> >>That's no good idea. If a disk fails which contains a swap partition to
> >>which data has been swapped the system will crash.
> >>Therefor it is important to use mirrored partitions for swap.
-- 
Raz
Long Live The Penguin

Re: RAID1: What happens when one half of a mirror fails?

[Dan Stromberg]

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On Thu, 2005-06-30 at 21:53 +0200, Frank Wittig wrote:
> Eric Pretorious wrote:
> > On Thursday 30 June 2005 12:03 pm, Laurent CARON wrote:
> > 
> >>is the swap partition on raid?
> > 
> > 
> > No. swap both swap partitions are formatted as standard swap partitions and mounted via /etc/fstab.
> > 
> 
> That's no good idea. If a disk fails which contains a swap partition to
> which data has been swapped the system will crash.
> Therefor it is important to use mirrored partitions for swap.

This is totally specific to the usage of the machine you're doing the
mirroring on: the relative importance of loss of disk space vs downtime.

If a little downtime is no big deal, and you could use a little extra
disk space, then sure, don't mirror swap.

If downtime is more important than the loss of a little disk space, then
do mirror swap.

Before you say that disk is always cheaper than downtime, recall that
sometimes there'll be a layer of bureaucracy between you and the
purchase of a new disk.


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Re: RAID1: What happens when one half of a mirror fails?

[Andy Smith]

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On Tue, Jul 26, 2005 at 09:30:09PM -0700, Dan Stromberg wrote:
> If a little downtime is no big deal, and you could use a little extra
> disk space, then sure, don't mirror swap.
> 
> If downtime is more important than the loss of a little disk space, then
> do mirror swap.
> 
> Before you say that disk is always cheaper than downtime, recall that
> sometimes there'll be a layer of bureaucracy between you and the
> purchase of a new disk.

But in that case if you mirror swap then you can have the pleasure
of working through the bureaucracy while the server is still up. :)

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