first pre-emptive raid

[Derek Ragona]

At 02:57 AM 6/28/2008, prad wrote:
>our dual pentium3 1GHz with 2G ram and 8 18G scsi drives (server holds
>4) should be arriving in about 1 week. my son and i want to this up as
>proper server rather than as a desktopish installation being used as a
>server. it will serve primarily websites (static html) and email for
>virtual domains as well as implement dns.
>
>from the handbook, we are learning
>
>1. how to installing scsi drives (we have some old 2G) from #18.3
>2. about software raid #18.4.1 (because we don't have a hardware
>solution and i guess you really don't have anything to figure out with
>one)
>3. about geom #19 and vinum #20
>4. about raid principles in general from wikipedia
>
>
>after a first reading, some initial questions about items:
>
>3. it seems that geom just does striping and mirroring, but vinum
>offers more configurability and is really the preferred choice?
>
>4.1 with 4 18G drives one thought is to do a raid1, but we really
>don't  want 3 identical copies. is the only way to have 2 36G mirrors,
>by using raid0+1 or raid1+0?
>
>4.2 another possibility is to do raid0, but is that ever wise unless
>you desperately need the space since in our situation you run a 1/4
>chance of going down completely?
>
>4.3 is striping or mirroring faster as far as i/o goes (or does the
>difference really matter)? i would have thought the former, but the
>handbook says "Striping requires somewhat more effort to locate the
>data, and it can cause additional I/O load where a transfer is spread
>over multiple disks" #20.3
>
>4.4 vinum introduces raid5 with striping and data integrity, but
>exactly what are the parity blocks? furthermore, since the data is
>striped, how can the parity blocks rebuild anything from a hard drive
>that has crashed? surely, the data from each drive can't be duplicated
>somehow over all the drives though #20.5.2 Redundant Data Storage has
>me scratching my head! if there is complete mirroring, wouldn't the
>disk space be cut in half as with raid1?
>
>
>this is all very interesting and very new to us.
>
>
>--

Striping alone offers speed but no data protection.

Mirroring offers redundancy but uses twice the disk space, AND is slower 
than striping.

Mirror + striping offers the best of both speed with redundancy.  However 
this configuration requires drive arrays of at least 4 drives and usually 
drives are added in 4's.  For complete safety you should have two drives in 
the array as hot spares as you can lose two drives.

Raid 5 attempts to offer data protection via saving parity checksums in 
another location.  However, it is possible to have both the data area fail, 
and the parity fail, making a rebuild impossible.  This can happen if you 
have two drives fail.  In newer hardware offering RAID 6, the parity is 
saved to 2 different drives, making the failure less likely.

I would suggest you wither do mirrored, or mirrored + striped  as these are 
older drives you are using.

For those of us that have lost drives in various ways, they die on their 
own of age, power problems will kill drives (from bad AC power AND/OR bad 
power supplies), heat of course will kill drives, etc.  Since most drives 
are installed at the same time often from the same manufacturer's lot, if 
there is any sensitivity or defect, you can easily lose multiple drives 
that way as well.

         -Derek



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