Re: how to synchronize two devices (RAID-1, but not really?)

[Bill Davidsen <davidsen@tmr.com>]

David Greaves wrote:
> Tomasz Chmielewski wrote:
>   
>> Peter Rabbitson schrieb:
>>     
>>> Tomasz Chmielewski wrote:
>>>       
>>>> I have a RAID-10 setup of four 400 GB HDDs. As the data grows by several
>>>> GBs a day, I want to migrate it somehow to RAID-5 on separate disks in a
>>>> separate machine.
>>>>
>>>> Which would be easy, if I didn't have to do it online, without stopping
>>>> any services.
>>>>
>>>>
>>>>         
>>> Your /dev/md10 - what is directly on top of it? LVM? XFS? EXT3?
>>>       
>> Good point. I don't want to copy the whole RAID-10.
>> I want to copy only one LVM-2 volume (which is like 90% of that RAID-10,
>> anyway).
>>
>>
>> So I want to synchronize /dev/LVM2/my-volume (ext3) with /dev/sdr (now
>> empty; bigger than /dev/LVM2/my-volume).
>>
>>
>> (sda2, sdb2, sdc2, sdd2) -> RAID-10 -> LVM-2 -> my volume -> ext3
>>
>>
>>     
>
>
> I've not used iSCSI but I wonder about using nbd : network block device
>
> Use nbd to export /dev/md5 from machine 2.
> Import /dev/nbd0 on machine 1.
> Add nbd0 to the VG on machine 1
> pvmove the data from /dev/md10 to /dev/nbd0 (ie the md5 on machine2 via nbd)
> remove /dev/md10 from the VG.
> The VG should now exist only on /dev/nbd0 on machine 2
> stop the services and lvm on machine 1
> start the lvm and services on machine 2.
>
> I'd suggest testing this first <grin>.
>   

Posts to the kernel mailing list indicate that people, including me, 
have had issues with this since 2.6.10-rc versions. I got no "me too" 
responses, so I figured something have changed, although yesterday 
someone reported a hard crash using nbd.

Test it well, but it's the ideal solution, combined with a bitmap, for 
getting in sync.

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