Re: RAID questions

[Jake Morrison <jake_morrison@example.com>]

Jc,

One thing to keep in mind is peformance. RAID 5 can be 
significantly worse in write performance than RAID 1 (or no RAID),
though read performance is potentially better. With RAID 5, 
when you write anything to disk, it is necessary to read from 
all the other disks in the set to get the parity information, 
then write to the target disk and the parity disk. 

In your case, it probably doesn't make that much difference,
but large mail servers are usually disk IO bound, so they can't
use RAID 5. Mail servers (and database servers) are usually set
up with RAID 1+0 (mirroring, then striping).

Regards,
Jake


--- "A.Sajjad Zaidi" <sajjad@example.com> wrote:
> Jean-Christian Imbeault wrote:
> 
> > Ok, I've been doing some reading on RAID. I think I have the basics
> > down but a few questions came up. Could some one clarify 
> the following for me?
> >
> > 1- What's the advantage of RAID level 1 versus level 5? They 
> > both seem to offer redundancy.
> 
> RAID level 1(mirroring) only offers redundancy. That is, with 2
> drives using RAID level 1, you only get the capacity of one drive.
> 
> RAID 5 offers this as well as striping (concatinating 2 or more
> drives to form 1 large drive). So with 3 drives, you can have 
> the space of 2 as well as redundancy.
> 
> With both, you can set the system up so if one disk fails, another
> can take over.
> 
> > 2- Is it still the case that you have the whole system under RAID.
> This
> > because having system files and a bootable RAID system is
> contra-indicated.
> > (Or at least causes so much sysadmin pain it's not worth it)
> 
> Not sure what you mean, but there is no problem with having the whole
> system on
> RAID. Your boot loader and kernel should support booting off of RAID
> though.
> 
> > 3- How do you know when a disk has failed?
> 
> I run a script every minute to check /proc/mdstat for any error
> messages.
> 
> > 4- How easy is it to replace a failed disk and have the system back
> up?
> 
> If the system support hot-swapping, all you need to do is pull out
> the bad
> drive and replace it with a good one. Assuming your system is
> configured
> correctly, you wont have any downtime.
> 
> If it doesnt support hot-swapping, youll need to power off and do the
> replacement.
> 
> > I'm thinking of setting up a mail server at out office and I was
> thinking
> > that since it is somewhat of a mission critical system installing
> RAID would
> > be a good idea. To protect against disk failure I tought RAID would
> be a
> > good idea. But maybe there are better ways than RAID?
> 
> The more redundancy the better. You can have RAID and do regular
> offsite
> backups.
> 
> Regards,
> 
> 41 53 5A
> 
>
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