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Re: RAID (was: "rock solid" motherboard)

> > This is just plain wrong. I suggest you go and read some good
> > documentation on the properties of different RAID formats. Generally
> > speaking RAID0 doubles throughput for large writes as data is striped
> > across both volumes, and has seek times the same as a single drive. RAID1
> > (mirroring) usually gives similar throughput to a single drive for large
> > transfers, and a good driver can reduce average seek times by reading
> > data from whichever drive head is nearest.
> When would the heads ever not be in the same place on a raid1 setup?  I
> would certainly be happiest if the drive always read both disks and
> compared the reads to detect problems (but then again if it did, which
> one of two different values would be correct?).  I would have thought
> the heads were mostly in sync at all times.

Consider you have two processes reading for different files. The kernel can 
read both files in parallel using one drive for each. The heads will only 
tend to be aligned after a write, even then the kernel could write data out 
to the drives in a different order.

RAID1 does not provide any consistency checking, or protection against data 
corruption. The only thing it protects against is total and catastrophic 
drive failure. The same is true of raid4 and raid5.

One of the advantages of software raid is that the kernel can optimise it's IO 
workload to minimise head movement. This is also possible with hardware 
controllers if they support command queueing (TCQ/NCQ). I've no idea if 
dmraid does this or not.

> > Not true. Read http://linux.yyz.us/sata/faq-sata-raid.html.
> > Most (maybe all?) of the proprietary raid formats are supported by the
> > dmraid driver.
> But is proprietary software raid ever faster/more efficient than linux
> md raid? 

No. The only advantage is that if you dual boot you can use the same raid set 
in windows. Normal md raid is more flexible, better tested.

> And what if your board dies and you have to move the drives to 
> another machine, how do you get the raid back?

I think the dmraid drivers will work with any controller, though I've never 
tried it.


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