[CentOS] Centos 7 RAID tutorial?

Mon Sep 8 03:09:09 UTC 2014
Digimer <lists at alteeve.ca>

On 07/09/14 10:43 PM, Valeri Galtsev wrote:
> On Sun, September 7, 2014 9:30 pm, John R Pierce wrote:
>> On 9/7/2014 7:19 PM, Dave Stevens wrote:
>>> I want to set up a new CentOS install using version 7 and would like
>>> to experiment with various RAID levels. Anyone care to point out a
>>> tutorial?
>> how many drives do you have for this experiment?  whats the target
>> usecase for the file systems on the raid(s)?   whats the level of data
>> resiliance required by said use case?
>> Raid only protects against one specific sort of failure, where an entire
>> disk drive fails.   It doesn't protect against data corruption, or
>> system failure,
> Even more: system failure or power loss is more likely to destroy all data
> on software RAID than on a single drive when there is a lot of IO present
> (to the best of my understanding, loss of cache software RAID is using is
> more catastrophic compared to journaled filesystem under same
> circumstances - somebody may correct me). So, there may be worth thinking
> about hardware RAID.
> Just my 2c.
> Valeri

Valeri makes an excellent point, which I would like to elaborate on;

Hardware RAID *with* flash-backed/battery-backed cache. I find it 
endlessly fascinating how many machines out there have hardware RAID 
with BBU/FBU. When using write-back caching without a battery leaves you 
in no better position.

Note that if you do get hardware RAID with BBU/FBU, be sure the cache 
policy is set to "Write-Back with BBU" (or your controllers wording). 
The idea here is that, if the battery/caps fail/drain, the controller 
switches to write-through (no) caching.

I'm not so familiar with software RAID, but I would be surprised if 
there isn't a way to force write-through caching. If this is possible, 
then Valeri's concern can be addressed (at the cost of performance).


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