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Re: [K12OSN] Linux "Software RAID"



[LOTS OF SNIPS]
>>>> I hear people extolling the virtues of "software RAID" on the list a
>>>> lot.  I'm finally setting up a production server in a school and I have
>>>> enough disks to play with to do RAID.  I'm leaning towards RAID 5.  Anyway,
>>>
>>> If you're thinking of RAID 5, which is my preferred level, I'd avoid
>>> doing it in software and instead opt for a dedicated RAID card.  Something
>>> like an LSI MegaRAID 150-6 SATA controller.  If you do it in software,
>>> you'll eat up some CPU doing the parity calculations, so you definitely want
>>> to offload that.  However, for just mirroring (say, RAID 1), you should be
>>> fine, because the CPU hit for mirroring is minimal.
>>
>> I hear lots of people talk about the CPU hit of software RAID.  But how
>> much hit is there really?  Suppose for argument's sake I can get a hardware
>> RAID card for $100.  If I instead used software RAID and spent my $100 on a
>> better CPU, wouldn't I be ahead of the game?
>
> No, I don't believe so.  For one thing, as Dan Young put it, it's much
> easier to deal with swapping a failed disk out with a dedicated card.  That
> by itself is a *BIG DEAL*.  Additionally, if you do have a disk fail, your
> CPU will take an especially big hit, because then it's got to reconstruct
> data from the parity info for *all* disk accesses, not just writes.
>  Oops....
>
> Furthermore, you don't have to depend on the OS for reading your RAID.  As
> long as it's a well-known FOSS-supported card, you can slap it into a
> FreeBSD, Net/OpenBSD, Linux, MS Windows, probably even Apple's Mac OS X.
>  Much more flexibility.  This has saved my butt before.

Hmm.. I have been using and advising software raids simply because I
do not know of any FOSS programs / enabled cards that will monitor and
report RAID status. Non-FOSS solutions run on Window$.

So how do you know about RAID status? I find that pretty easy using
mdadm and as Rob put it I hardly see any CPU overhead. Yes it is there
when you are re-bulding arrays but I have done disk swap on a running
machine and rebuilt RAID (level 1) without any user complaining of
LTSP slow down.

I would appreciate if a hardware RAID resource list can be compiled
for RAID cards and monitoring software.

-- 
Regards,
Sudev Barar
Read http://blog.sudev.in for topics ranging from here to there.

PS: I know most of people do not follow email niceties (mostly they
are not aware) but if you follow bottom post/in-line post style of
email conversations it becomes a whole lot easier to carry on
meaningful dialogue and you can snip out what is not meaningful too.
Most people just hit reply button and top post leaving prior message
appended uselessly at bottom. See if you can adopt this style and
persuade others. In case you are already doing this ..... great,
spread the message.


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