[Beowulf] hardware RAID versus mdadm versus LVM-striping

[Joe Landman]

Rahul Nabar wrote:
> On Sun, Jan 17, 2010 at 9:36 PM, Joe Landman
> <landman at scalableinformatics.com> wrote:
>> Ohhh ... it depends.  Some of the "vendors" hardware raid ... heck ... most
>> of it ... is rebadged LSI gear.  Usually their lower end stuff which is
>> sometimes fake-raid.  Use fake-raid only if no other options exist.
> 
> Thanks Joe! What's "fake RAID"? Just a bad implementation or...........?

Its a software RAID implementation pretending to be a hardware RAID 
implementation.  They are rarely if ever as good as MD.  Many of them in 
Linux will invoke dm (the "other" RAID engine) as dm has "support" for 
fake-raid.  Note that we have lost data (multiple times) with 
dm+fake-raid in testing, so we don't recommend its use in important 
machines (ones which you can't afford to lose).  This could be due to 
bad drivers for the chips in question, but we aren't taking chances.

> 
> 
>> LVM is not a performance tool.  Use it to help you manage things, not speed
>> things.
> 
> I had thought so. But why then does LVM have features like striping if
> not for performance? Or are they just not so good?

LVM doesn't perform as well as MD RAID for performance.  You can use it, 
just be advised that you are leaving a great deal of performance on the 
table if you do so.


> 


-- 
Joseph Landman, Ph.D
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