[mythtv-users] anyone using software RAID 5 for 1 terabyte +

[John Drescher]

On 8/6/06, mark <list at onnow.net> wrote:
> I need to build some serious space in a FC5 box.
> I am thinking 4 or 5 250 Gig drives in RAID5 using mdadm ( software RAID )
>
> Anyone using this or a similar config?  How is the performance?
>
At work I have several 1TB+ software raid 5 and raid 6 arrays.  The
read performance is almost as good as raid0, I mean on a new 6 drive
raid 6 system we have benchmarked the STR at 267MB/s. Write
performance is definitly better than a single disk but we have no real
numbers.

> The only bad thing about the raid 5 is that it cant be added to.  Not
> sure how to address that one.
>
This is being added to the latest kernel (if it is not already in).
This new ability is called raid5_reshape. Actually some parts of this
are in, but the ultimate goal is to be able to upgrade from raid 0 ->
raid 5 -> raid 6 and add disks to any of these arrays and to also be
able to change the size of the disks.

John

> Thoughts?
>
You did not mention any pc specs as I have seen this makes a big
difference. I have a 8 drive pata raid 6 system with 8 120GB WD drives
on a dual processor AthlonMP with 2GB of memory and this system only
gets ~80MB/s even though the drives are connected to the pci-x bus and
each drive has its own channel. The 267MB/s number was take on a new
single cpu athlon64 3200 am2 M2N-SLI deluxe with 6 seagate 7200.10 330
GB drives.

Other thoughts are in any case on any athlon 2000 or better system I
never see cpu usage of more than around 10% when writing to the disk
using raid 6 (which is raid 5 with two sets of parity instead of one
and two drives can fail with no data loss). However during rebuilds
the system power makes a large difference. The M2N system rebuilded at
40 to 60MB/s while the older athlonmp rebuilded at 5 to 10 MB/s.

John