Re: partitioned mirror vs. mirrors of partitions?

[Bill Davidsen <davidsen@tmr.com>]

Eric Sandeen wrote:
> Bill Davidsen wrote:
>   
>> Eric Sandeen wrote:
>>     
>>> Hi, I hope this isn't a FAQ, I did do a little searching first...
>>>
>>> I'm looking at using a couple of large disks to mirror a system which
>>> currently has a few different filesystems; I'll use partitions on the
>>> disks to contain the different fileystems.
>>>
>>> It looks like I could mirror sda and sdb, and partition the resulting
>>> md_d0.  Or, I could partition sda and sdb, and create mirrors md0, md1,
>>> etc from the partitions on the underlying disks.
>>>
>>> Is there any technical reason to choose one method vs the other?  It
>>> seems to me that perhaps on a system with several active partitions from
>>> the same disk, partitioning a single large raid device might allow
>>> better read balancing?
>>>   
>>>       
>> The reason for going with a partitioned raid is that rebuild after a 
>> failure is easier. The reason for NOT going there at the moment is 
>> discussed in another thread here, in the current kernel the partitions 
>> are not started unless you have an initrd file to make that happen. The 
>> last is performance, if you are using the partitions in different ways, 
>> and some would benefit from performance while others (/boot comes to 
>> mind) need to be simple and reliable, and have minimal requirements for 
>> speed. Having partitions on the drive allows you to use different raid 
>> levels across partitions, to best fit what you do with that data.
>>     
>
> Thanks.  In my case I'd just have raid-1 on everything, so don't need
> that granulatiry...  Another drawback in my particular case is that the
> Red Hat / Fedora tools don't seem to grok partitioned md, but I can fix
> that ;)
>
> Is there any merit to my notion about better read balancing across the
> entire disk if it's all one md device?
>   

Not that I can see, but that doesn't mean you're wrong, just that I 
can't think of any reason why the same load on the same drives would be 
better balanced.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark