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Re: [K12OSN] RAID in general ... was ... Software RAID... longmessages



Also note that mirrored drives probably have the best performance, because
requests can be serviced by either the drive or the mirror.  In addition,
there is no parity slowdown.

Also note that RAID does not shield you from data _corruption_ only
complete drive _failure_.

Jon

On Sat, 7 Dec 2002, Bert Rolston wrote:

> Bert here,
>
> This isn't a definitive guide to RAID either, only what I can remember.
> This information could contain some discrepancies.
>
> >
> > <snip>...The problem is not the drive, but the IDE interface.
>
> As others have experienced IDE performance starts to degrade as the
> number of users increases. This is anecdotal evidence, but if all the
> major manufacturers use SCSI there is likely to be a good reason. (I can
> hear the skeptics saying "yeah $" even now).
>
> >
> > >
> > >Does anyone have any experience with linux and software RAID?  I make
> > >nightly backups, so short of HORRIBLE un-reliability, I'm not overly
> > >concerned about doubling my chances of a hard drive failure.  Is the
> > >difference in speed noticable?  Does it put a big load on the CPU?  Does
> > >striping help more with large files, or small files?  Both?
> > >
>
> Software RAID will load up the CPU. For a small number of users this MAY
> be acceptable.
>
> Hardware RAID is preferable. Hardware controllers have their own
> processor on card so the server cpu is free to do server stuff.
>
> Some older RAID controllers used the INTEL 80186. The newer controllers
> I have encountered appear to use a MOTOROLLA processor, but I haven't
> seen every make and model available.
>
> Striping will improve performance, because you get multiple simultaneous
> reads and writes.
>
> Think of it like this -
>
> You have 8 bits of data and 8 drives (for striping eg RAID 0,4,5)
>
> Instead of writing the data to a single drive serially, you now get the
> data written ACROSS 8 drives in parallel, IE bit1 to drive1, b2 to d2...
> and so on.
>
> What happens when a drive fails in a stripe set?
> This depends on the RAID level used.
>
> RAID 0 = stripe set, no parity.
> Loose 1 drive = disaster
>
> RAID 4 = STRIPE set w/parity bit on separate drive.
> Loose 1 drive = no problem unless it is the parity drive.
> This level tends to thrash the parity drive therefore it has a higher
> failure rate then the data drives.
> This level can carry on with a dead data drive.
> It isn't used very much because of the parity drive failure rates.
>
> RAID 5 = stripe set w/parity bit distributed across drives.
> Loose 1 drive = no problem, data and parity is distributed. This level
> can carry on with one dead drive.
> If you have hot swap drives and the controller is capable you can change
> a drive without downing the server. I've seen this done successfully on
> Compaq servers running NT or Netware.
>
>
> RAID 1 is a separate kettle of fish. This is drive mirroring.
> Bit1 is written to d1 & d2, b2 to d1 & d2, and so on. Serial data writes
> to parallel drives.
> Loose 1 drive, no problems.
> With hot swap drives this will rebuild on the fly as well.
>
> So what has this got to do with anything?
>
> Software RAID is usually confined to level 0 & 1.The other options
> require a lot of processor time controlling writes AND parity checking
> for each stripe. Distributing the parity bits across drives requires
> more processing.
>
> Here's the cool part, you can combine RAID levels IE RAID 5+1. I'm not
> sure if this available in software.
>
> What is RAID 5+1?
>
> Two identical stripe sets with distributed parity that have been
> mirrored.
>
> Theoretically you can loose several drives and continue to operate with
> this configuration.
>
> > Seperating the issues ;
> >
> > Stripe/RAID will double your statistical chance of a HDD causing a
> > server failure - an unacceptable risk IMO.
>
>
> Not quite true. This is dependent on the RAID level / combination you
> use, AND the $ you have available. ;-)
>
> Putting it in very simple terms
>
> No parity = high disaster probability
> Mirror only / Parity only = lower disaster probability
> Parity + mirror = lowest disaster probability
>
>
> > RAID means *redundant* disks - not the other way around.
>
> R edundant
> A rray
> I ndependant
> D rives
>
>
> I know this was long but hopefully it will help.
>
> Cheers,
> Bert
>
>
>
>
>
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