[vdr] mdadm software raid5 arrays?

Jochen Heuer jogi-vdr at planetzork.ping.de
Tue Nov 10 20:24:39 CET 2009


Hello Simon,

what you also can do is to create the two RAID1 md devices with "missing"
disks, e.g.:

mdadm --create /dev/md2 --level=1 --raid-disks=2 missing /dev/sdb3
mdadm --create /dev/md3 --level=1 --raid-disks=2 missing /dev/sdd3
mdadm --create /dev/md1 --level=0 --raid-disks=2 /dev/md2 /dev/md3

Then you can create a filesystem on /dev/md1 and mount it, move all your
recordings to that filesystem and lateron you can add the other two partitions
to your RAID1 sets:

mdadm --add /dev/md2 /dev/sda3
mdadm --add /dev/md3 /dev/sdc3

This way you don't have to split anything. You can just setup the two RAID1
arrays with one only one drive and one drive missing.

All of my systems (except of VDR because it currently only has one disk) has a
mirrored / and grub installed on both disks.

I don't know if you can read German or if a Google translation of the following
pages is usable but it might help you to get the correct keywords for a Google
search:

http://linuxwiki.de/mdadm
http://www.howtoforge.de/howto/software-raid1-auf-einem-laufenden-system-inkl-grub-konfiguration-debian-etch-einrichten/

Best regards,

   Jogi

On Wed, Nov 11, 2009 at 07:48:20AM +1300, Simon Baxter wrote:
> Thanks - very useful!
>
> So what I'll probably do is as follows...
> * My system has 4x SATA ports on the motherboard, to which I'll connect my 
> 4x 1.5TB drives.
> * Currently 1 drive is in use with ~30G for / /boot and swap and ~1.4TB for 
> /media
> * I'll create /dev/md2, using mdadm, in RAID1 across 2 ~1.4TB partitions on 
> 2 drives
> * move all active recordings (~400G) to /dev/md2
> * split /dev/md2 and create a raid 1+0 (/dev/md1) using 4x partitions of 
> ~1.4TB across 4 drives
>
> At this point I have preserved all my data, and created a raid1+0 for 
> recordings and media.
>
> I should now use the remaining ~100G on each drive for raid protection for 
> (root) / and /boot.  I've read lots on the web on this, but what's your 
> recommendation?  RAID1 mirror across 2 of the disks for / (/dev/md0) and 
> install grub (/boot) on both so either will boot?
>
>
>> On Tue, Nov 10, 2009 at 09:46:52PM +1300, Simon Baxter wrote:
>>> What about a simple raid 1 mirror set?
>>>
>>
>> Ok.. short comparison, using a single disk as baseline.
>>
>> using 2 disks
>> raid0: (striping)
>> ++   double read throughput,
>> ++   double write throughput,
>> --   half the reliability (read: only use with good backup!)
>>
>> raid1: (mirroring)
>> ++   double read throughput.
>> o    same write throughput
>> ++   double the reliability
>>
>>
>> using 3 disks:
>>
>> raid0: striping
>> +++  tripple read performance
>> +++  tripple write performance
>> ---  third of reliability
>>
>> raid1: mirroring
>> +++  tripple read performance
>> o    same write throughput
>> +++  tripple reliability
>>
>> raid5: (distributed parity)
>> +++  tripple read performance
>> -    lower write performance (not due to the second write but due
>>      to the necessary reads)
>> +    sustains failure of any one drive in the set
>>
>> using 4 disks:
>>
>> raid1+0:
>> ++++ four times the read performance
>> ++   double write performance
>> ++   double reliability
>>
>>
>> please note: these are approximations and depending on your hardware
>> they may be off by quite a bit.
>>
>> cheers
>> -henrik
>>
>>
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>
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