New Webserver with multiple drives

[Matthew Seaman]

On Sun, Jul 27, 2003 at 06:20:27PM -0500, Duane Stark wrote:

> To preface this, I'm not OS retarded - just BSD retarded ;)  I haven't had to mess with my current BSD server since I bought it, and now I have purchased a new p4 3.0(something), 2 gig ram, 2 IDE 200gig HD's to replace it..
> 
> Here is my question:
> 
> How do I setup these multiple drives?  What does the "industry" recommend when it comes to setting them up?  Should I set BSD up to think its one datasource (so 400gig) and then run from that? Or do I setup 1 drive to hold my web/mail/mysql, and the other to do something elsE?
> 
> I'm totally lost, so any help would be greatly appericated.. PLEASE don't assume I know what your talking about, because it's a given that I dont! heh :)

The only possible answer is "it depends".  With disks there are 3
characteristics that you can modify the balance between depending on
your needs.  Those are resilience, available space and access speed.
There's also a fourth consideration, which may affect your choice but
that has little effect during the day-to-day operation of the system,
which is the amount of time and effort you're prepared to put into
doing sys-adminly things.

Now, you've only got two disks, so that immediately rules out any
choices involving RAID5.  You make no mention of any sort of hardware
raid controller, so I'll assume that isn't a possibility either.

That leaves essentially 3 choices:

   i) No RAID at all.  This scores highly on the ease of admin, as
      it's the default way things are set up by sysinstall.  Just
      partition the disks, put filesystems on them and set up
      /etc/fstab so the partitions get mounted in appropriate
      locations.  I'll take this as the baseline to compare the other
      setups to.

  ii) RAID 0 or disk striping.  This creates one synthetic 400Gb
      partition from your two actual drives, by writing alternate
      blocks of data to each drive.  The block size is configurable:
      at one extreme you could make the block size the same as the raw
      disk size, in which case you'ld end up appending one disk to the
      end of the other.  However, the greatest advantage occurs when
      the block size is round about the same size as the system can
      read from the drive in one gulp.  This spreads the load of any
      IO evenly of the two drives and should maximize performance.

      The bad news is that if either of the disks becomes faulty, then
      all of the disk space on your system will be unavailable.  As
      you add disks to the stripe, this problem becomes more and more
      acute, so this setup is generally not used very much unless in
      combination with RAID 5 or RAID 1 to give higher resilience.

 iii) RAID 1 or mirroring.  Each drive contains a complete copy of all
      of the data, maintained in parallel.  The advantages are
      improved resilience -- the system should just keep chugging
      along merrily even if one of the drives self destructs -- and
      improved IO performance on reads -- writes have to go to both
      drives, which takes only slightly longer than writing to a
      single drive, but reads can go to either drive which gives you
      much better performance.  (The biggest factor is the
      milliseconds it takes to position the head and wait for the
      drive to turn round until the correct block is under the head.
      Talking between the CPU, RAM and the disk electronics takes of
      the order of microseconds.)

      The bad news is that you've got to sacrifice half of your
      potentially available disk capacity.  However, assuming that the
      resulting space is adequate for your needs, a mirrored root disk
      setup is pretty standard for server machines.

Either of ii) and iii) will probably entail your learning about
vinum(8) as the best available mechanism for doing software RAID on
FreeBSD.  The alternatives are not that hot: ccd(4) is pretty ancient
and doesn't offer any means of recovering a mirrored partition than
backup and re-install should one drive fail.  I've heard that NetBSD's
raidframe stuff is being ported to FreeBSD, but I don't think it's
ready for primetime use yet.

See
http://www.freebsd.org/doc/en_US.ISO8859-1/articles/vinum/index.html
for a thorough introduction to vinum bootstrapping,
http://www.vinumvm.org/ for general information and
http://org.netbase.org/vinum-mirrored.html for a quick HOWTO set up a
bootable vinum root drive.

	Cheers,

	Matthew	

-- 
Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP: http://www.infracaninophile.co.uk/pgpkey         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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