Re: [PLUG] Mirrored drives with LVM

On Sat, Jun 7, 2008 at 1:08 PM, James Barrett <jadoba@jadoba.net> wrote:

If you are considering four RAID5 arrays, you might want to try two
RAID6 arrays instead (if the hardware controller permits)

So, 12 disks, 500GB each, RAID5 or RAID6...

{{{crunches numbers}}}

2 full raid5 arrays would yeild 5TB of space
([{# of disks in a single array, -1} multiplied by amount of space on
each disk]multiplied by # of arrays).

2 full RAID6 arrays would yeild twice the disk failure allowance but
only 4TB of disk space
([{# of disks in a single array, -2} multiplied by amount of space on
each disk]multiplied by # of arrays)
If you want to set aside a disk in each array as a hot spare, remove
another TB of space in either instance.

Your question is about performance, and I'll tell you that you really
won't have much if using RAID5 or RAID6.

BUT if you had 6 mirrored RAID1 arrays you would end up with 3TB of
space without hot spares and some confusing redundancy.

So the question you must ask yourself is "do I want speedy IO, or more space?"

If you require lots of space, go for the RAID5. If you require space
but smarter and fuller redundancy, go with RAID6.

If you opt for the speedy IO then implement the six RAID1 arrays, and
then ask yourself "Do I feel lucky?"

--
Jim

On Sat, Jun 7, 2008 at 9:08 AM, Mag Gam <magawake@gmail.com> wrote:
> hey Jim
>
> thanks for the reply.
>
> My setup is very different from the OP. I was just intrigued by his LVM
> write up. I suppose I should start a new thread :-)
>
> I have 12disk (500GB) disk array. I would like to setup 2 (or 3 or 4) RAID
> 5 volumes via controller. Once I setup the volumes, I will create PVs out of
> them using LVM. The LVs create I plan on using striping and put ext3 fs. My
> question is, what would be the optimal setup for performance? Redudancy is
> important but would like to get good performace because we are planning to
> do scientific computing for the summer at my University...
>
>
>
>
> On Sat, Jun 7, 2008 at 8:53 AM, James Barrett <jadoba@jadoba.net> wrote:
>>
>> RAID on HW level - I am supposing you mean two RAID1 arrays? Striping
>> with LVM... as in, two physical devices, one VG, and striping...
>>
>> This is feasible. I do not see how doing this would be any different
>> than RAID1+0 or RAID0+1, allowing you to lose two of four disks. It
>> just depends on which disks you lose! If you lose two disks in the
>> same array, all data is lost. If you lose one disk in each array, you
>> still have all data. Of course, I imagine that using LVM for the
>> striping allows you to add non-raided disks, which completely defeats
>> the purpose of using any raided disks.
>>
>> If you choose RAID6 (2 parity), it will give different results. You
>> still have the ability to lose 2 disks, but without discretion on
>> which are lost. The downside of RAID6 is decreased performance.
>>
>> HTH
>>
>> --
>> Jim
>>
>> On Sat, Jun 7, 2008 at 8:30 AM, Mag Gam <magawake@gmail.com> wrote:
>> > Slightly off topic question ...
>> > I see you are using RAID on LVM, what if I am using RAID on HW level and
>> > I
>> > plan to stripe on LVM level. Is this a good practice?
>>
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