Superblocks

Superblocks

[Greg Cormier]

Can someone help me understand superblocks and MD a little bit?

I've got a raid5 array with 3 disks - sdb1, sdc1, sdd1.

--examine on these 3 drives shows correct information.


However, if I also examine the raw disk devices, sdb and sdd, they
also appear to have superblocks with some semi valid looking
information. sdc has no superblock.

How can I clear these? If I unmount my raid, stop md0, it won't clear it.

[root@zeus ~]# mdadm --zero-superblock /dev/hdd
mdadm: Couldn't open /dev/hdd for write - not zeroing

I'd like to rule out these oddities before I start on my next
troubleshooting of why my array rebuilds every time I reboot :)

Thanks,
Greg

Re: Superblocks

[Raz]

why you zeroing hdd ?  should you clear sdd?

On 10/26/07, Greg Cormier <gcormier@gmail.com> wrote:
> Can someone help me understand superblocks and MD a little bit?
>
> I've got a raid5 array with 3 disks - sdb1, sdc1, sdd1.
>
> --examine on these 3 drives shows correct information.
>
>
> However, if I also examine the raw disk devices, sdb and sdd, they
> also appear to have superblocks with some semi valid looking
> information. sdc has no superblock.
>
> How can I clear these? If I unmount my raid, stop md0, it won't clear it.
>
> [root@zeus ~]# mdadm --zero-superblock /dev/hdd
> mdadm: Couldn't open /dev/hdd for write - not zeroing
>
> I'd like to rule out these oddities before I start on my next
> troubleshooting of why my array rebuilds every time I reboot :)
>
> Thanks,
> Greg
> -
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>


-- 
Raz

Re: Superblocks

[Neil Brown]

On Friday October 26, gcormier@gmail.com wrote:
> Can someone help me understand superblocks and MD a little bit?
> 
> I've got a raid5 array with 3 disks - sdb1, sdc1, sdd1.
> 
> --examine on these 3 drives shows correct information.
> 
> 
> However, if I also examine the raw disk devices, sdb and sdd, they
> also appear to have superblocks with some semi valid looking
> information. sdc has no superblock.

If a partition starts a multiple of 64K from the start of the device,
and ends with about 64K of the end of the device, then a superblock on
the partition will also look like a superblock on the whole device.
This is one of the shortcomings of v0.90 superblocks.  v1.0 doesn't
have this problem.

> 
> How can I clear these? If I unmount my raid, stop md0, it won't clear it.

mdadm --zero-superblock device name

is the best way to remove an unwanted superblock.  Ofcourse in the
above described case, removing the unwanted superblock will remove the
wanted one aswell.


> 
> [root@zeus ~]# mdadm --zero-superblock /dev/hdd
> mdadm: Couldn't open /dev/hdd for write - not zeroing

As I think someone else pointed out "/dev/hdd" is not "/dev/sdd".

NeilBrown

Re: Superblocks

[Greg Cormier]

Which is the default type of superblock? 0.90 or 1.0?

On 10/30/07, Neil Brown <neilb@suse.de> wrote:
> On Friday October 26, gcormier@gmail.com wrote:
> > Can someone help me understand superblocks and MD a little bit?
> >
> > I've got a raid5 array with 3 disks - sdb1, sdc1, sdd1.
> >
> > --examine on these 3 drives shows correct information.
> >
> >
> > However, if I also examine the raw disk devices, sdb and sdd, they
> > also appear to have superblocks with some semi valid looking
> > information. sdc has no superblock.
>
> If a partition starts a multiple of 64K from the start of the device,
> and ends with about 64K of the end of the device, then a superblock on
> the partition will also look like a superblock on the whole device.
> This is one of the shortcomings of v0.90 superblocks.  v1.0 doesn't
> have this problem.
>
> >
> > How can I clear these? If I unmount my raid, stop md0, it won't clear it.
>
> mdadm --zero-superblock device name
>
> is the best way to remove an unwanted superblock.  Ofcourse in the
> above described case, removing the unwanted superblock will remove the
> wanted one aswell.
>
>
> >
> > [root@zeus ~]# mdadm --zero-superblock /dev/hdd
> > mdadm: Couldn't open /dev/hdd for write - not zeroing
>
> As I think someone else pointed out "/dev/hdd" is not "/dev/sdd".
>
> NeilBrown
>

Re: Superblocks

[Neil Brown]

On Tuesday October 30, gcormier@gmail.com wrote:
> Which is the default type of superblock? 0.90 or 1.0?

The default default is 0.90.
However a local device can be set in mdadm.conf with e.g.
   CREATE metdata=1.0

NeilBrown

Re: Superblocks

[Bill Davidsen]

Neil Brown wrote:
> On Tuesday October 30, gcormier@gmail.com wrote:
>   
>> Which is the default type of superblock? 0.90 or 1.0?
>>     
>
> The default default is 0.90.
> However a local device can be set in mdadm.conf with e.g.
>    CREATE metdata=1.0
>
>   

If you change to 1.start, 1.ed, 1.4k names for clarity, they need to be 
accepted here, as well.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: Superblocks

[Greg Cormier]

Any reason 0.9 is the default? Should I be worried about using 1.0
superblocks? And can I "upgrade" my array from 0.9 to 1.0 superblocks?

Thanks,
Greg

On 11/1/07, Neil Brown <neilb@suse.de> wrote:
> On Tuesday October 30, gcormier@gmail.com wrote:
> > Which is the default type of superblock? 0.90 or 1.0?
>
> The default default is 0.90.
> However a local device can be set in mdadm.conf with e.g.
>    CREATE metdata=1.0
>
> NeilBrown
>

Re: Superblocks

[Bill Davidsen]

Greg Cormier wrote:
> Any reason 0.9 is the default? Should I be worried about using 1.0
> superblocks? And can I "upgrade" my array from 0.9 to 1.0 superblocks?
>   

Do understand that Neil may have other reasons... but mainly the 0.9 
format is the default because it is most widely supported and allows you 
to use new mdadm versions on old distributions (I still have one FC1 
machine!). As for changing metadata on an existing array, I really can't 
offer any help.
> Thanks,
> Greg
>
> On 11/1/07, Neil Brown <neilb@suse.de> wrote:
>   
>> On Tuesday October 30, gcormier@gmail.com wrote:
>>     
>>> Which is the default type of superblock? 0.90 or 1.0?
>>>       
>> The default default is 0.90.
>> However a local device can be set in mdadm.conf with e.g.
>>    CREATE metdata=1.0
>>
>> NeilBrown
>>     


-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979