RE: Re: Just set up my first RAID 1 array : Questions

["Paul Gordon" <paul@xxxxxxxxxxxxxxxxxxxxx>]

OK, I'll make some general comments....

There is either "software RAID" or "hardware RAID" -
the difference
should hopefully be obvious, in that the software RAID does not require
any special controller hardware beyond perfectly bog-standard disk
interfaces, and the RAID setup is controlled entirely in software by the
host OS. Various versions of Windows support this. Hardware RAID
obviously does have dedicated hardware, which can be a separate
controller card, or it's quite common these days to find RAID features
built in on modern motherboards.

With hardware RAID, the answer to most of your questions will be: "it
depends" - apart from the actual RAID standards the implementation is
up
to the individual controller manufacturers.

In my experience, RAID 1 mirrors don't generally have a distinction such
as "master" & "mirror" - both drives are treated
equally, and both are
exactly the same. Either drive can fail, and the volume will remain
online and can be read and written to as normal. The mirror will
normally be rebuilt automatically when the failed drive is replaced. If
the failure occurs mid-write, then yes, in theory, the other drive
should complete the write successfully, and thus the failed drive should
then be rebuilt from the good drive, complete with the write that was in
progress (and of course any subsequent ones since the failure).

I say "in theory" in there, because a great number of things can
conspire to still cause data corruptions, - consider for instance the
effect of on-drive cache... most if not all modern drives have 8MB or
more of onboard cache memory where writes are recorded prior to being
committed to the disk media. Once the "write" is completed into
this
cache, it is usually considered complete by the disk controller & the
OS. If the disk fails whilst there is still uncommitted data in the
cache, then that data is likely to be lost. For this reason, drives used
in RAID arrays typically are configured not to cache writes in this way
(often this is referred to as the cache being configured as
"write-through" or "write-back"). Then there is the
question of whether
the RAID controller also has cache memory; and if it does, is it
battery-backed? - Of course, the effect of losing data from cache
*should* still be protected by having the other drive in the array,
which should still complete the write correctly to the physical disk....
even so, it's a risk not worth taking...

Your suggestion about mixing the on-board & add-in SATA ports would
suggest to me that you'll be limited to software RAID only, as I'm not
aware of controllers which do hardware RAID capable of integrating any
ports not directly under the control of the RAID chipset. Windows'
built-in software RAID would be able to do this, - but the SATA ports
should be configured for "normal" i.e. non-RAID operation when
being
used for software RAID. The alternative is not to use the onboard ports,
& install a hardware RAID controller with 4 or 8 ports instead.

I am not a RAID or storage expert BTW!! So someone else may choose do
disagree with any part of these comments...


HTH

Paul G.


> -----Original Message-----
> From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf
Of
> noughtomate
> Sent: 13 October 2008 11:50
> To: UKHA Group
> Subject: [ukha_d] Re: Just set up my first RAID 1 array : Questions
>
> Pretty please ?
>
> --- In ukha_d@xxxxxxx, "noughtomate"
<balraj_jassal@...>
> wrote:
> >
> > Hi,
> >
> > This is the first time I've set up a RAID 1 mirror based on 2
SATA
> > connectors on my Mini-ITX motherboard. I would like to do some
> > testing against it.
> >
> > All the info I can find on the web is about the RAID standards,
> but
> > I want to know how a re-build is performed on failure. One of the
> > disks is named a 'Master' and the other the 'Mirror'. If I were
to
> > pull one out during mid-write, would any lost changes be auto-
> > propogated to the other when it's re-plugged in ?
> >
> > Is a re-build only performed by a Master ?
> > If the Master goes down, is the Mirror allowed to be written to ?
> > How does the BIOS know which has the latest data if both have had
> > independent writes from each other ? Do they merge into one ?
> >
> > Also, would anyone know of any testing tools I could use to cover
> > off all scenarios (Windows based)
> >
> > I just need some peice of mind, before I actually place my data
on
> > the disks.
> >
> >
> > Oh : one other question. Despite my Mobo's RAID compabilities,
I'm
> > only restricted to 2 SATA connectors. I would like to expand
this.
> >
> > Could anyone recommend a PCI Express card which supports JBOD,
> RAID
> > 0 and RAID 1, with 2 internal SATA connectors and ideally an
eSATA
> > connector for future expansion ? (The less depth the better)
> >
> > There seem to be many out there, but I'd prefer to hear about
ones
> > you use and would recommended.
> >
> > Thanks.
>


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