[LAU] drive bays and hardware RAID

[Florian Faber]

Arnold,

> And now imagine plutek checking ebay for days to hunt down the same hw-
> controller as the one that just fried up...

I am not arguing pro cheap shit. I am arguing against a general advice
to use software RAID, as there a numerous things that just don't work
with a software RAID - I'm talking about actually getting things done,
not theory.

> Oh, most people have two or more audio-interfaces. And two or more headphones 
> and speaker-pairs. And two or more guitars (if they are guitar-players).

What if his mainboard breaks, and he is using a PCI or Firewire
soundcard? And there's no mainboard with PCI slot or Firewire around
anymore?

Your argument 'if you buy a hardware RAID controller, buy a second one'
is simply not true. Yes, there are a lot of cheap solutions out there
and a lot of people got burned. But in general, this is as true as
'firewire is dead' :)

>> With software raid, you have to transfer all data twice through the
>> system,
> No, the only place where it feeds through twice is the part memory->disk-
> controller (which is pretty fast thanks to dma). Before that its just one 
> stream, after that its one dedicated bus per device (given this centuries 
> sata/sas).

For 0/1, yes.

>> you fuck up you caches,
> If the linux kernel fucks up its caches when its doing its own raid, something 
> is _horribly_ wrong. And I doubt it is.

If you only look at 0/1, of course not.

>> and the CPU has to do all the
>> computations. If you only have two drives, it doesn't matter that much.
> What computations? - Ah, you still don't know that any [345]-raid is bad...

Arnold, keep cool. There's more than 01345. And for good reasons.

>> So it really depends on what you want to do and what your budget is. If
>> you have to avoid IO stress, go and get a decent RAID controller.
> If you want to avoid user-stress, do software-raid.

It all depends on what you are doing. There is no general rule of thumb.
If you are dealing with professional video for example, you have very
high demands on bandwidth and reliability. If you want to achieve the
same reliability and performance you can easily get with a RAID 6(0)
with any combination of 0 and 1, you are wasting a huge pile of money.

>> This is only true if the on-disk format is not specified somewhere. If
>> you buy cheap shit, that may be and you may have to fiddle around to
>> extract the data. But it can be done.
> Again, the question is probably not so much the "if" or "how" but the "how 
> fast" and "how pricey".

Yes. The only thing that got me started was your general advise not to
use a hardware RAID. I'm all with you to avoid cheap hardware RAIDs. But
a proper hardware RAID is pure bliss.

> I know business you have lost raid5-systems (and all the data on it) because a 
> second disk broke under the stress of reconstructing a first failed disk.

We don't have to argue about the stupidity of RAID 5, specially with
nowadays disk sizes, or lazy administrators, that don't do integrity
checks on a regularly basis. For above mentioned setups, I use RAID 6(0)
with hot spares.

> But be my guest, I (*) work in a company helping such firms after disaster. 
> Note that we also write invoices for our work.

That is very fortunate for you. In that case of course you know that it
is almost always the user (administrator) that fucked up. Either by
chosing a wrong setup, or by making a minor error worse when taking
wrong actions.


Flo
-- 
Machines can do the work, so people have time to think.
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