[vdr] mdadm software raid5 arrays?

Simon Baxter linuxtv at nzbaxters.com
Tue Nov 10 19:48:20 CET 2009


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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