[vdr] mdadm software raid5 arrays?

Alex Betis alex.betis at gmail.com
Thu Nov 12 13:03:29 CET 2009


Simon,

Pay attention that /boot can be installed only on a single disk or RAID-1
where every disk can actually work as a stand alone disk.

I personally decided to use RAID-5 on 3 disks with RAID-1 on 3xsmall
partitions for /boot and RAID-5 on the rest.
RAID-5 also allows easier expansion in the future.


On Tue, Nov 10, 2009 at 8:48 PM, Simon Baxter <linuxtv at nzbaxters.com> 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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