Re: RAID-5 vs. Disk Mirroring

From: Albert W. Dorrington <awdorrin_at_mail.delcoelect.com>
Date: 1996/01/31
Message-ID: <4eoea3$d06_at_kocrsv08.delcoelect.com>#1/1

mstrong_at_wotangate.sc.ti.com (Mike Strong,713 274 3317,0883922) writes:
|> We just made the decision to go with striping/mirroring instead of RAID-5. The RAID-5
|> performance issues were one major reason. The other reason is that in
|> order to add
|> drives to a RAID-5 array, the database needs to be exported, the new
|> drives added,
|> the disks reformatted, and the database imported (this is according to
|> a SUN rdbms
|> guru and our system admin; don't yell at me if this isn't correct).
|> We did not want
|> to take the downtime hit for the export/install/format/praying/import
|> /praying.
|>
|> My boss was not happy with the disk cost (1G = 500M mirrored), but he
|> really did not
|> like the downtime costs.
|>

        This all depends on how you are going to utilize your RAID arrays and how you will expand in the future, so the above is not necessarily true.

        For example, I'll use the configuration for our site.

We have two RAID controllers, each of 10GB - consisting of seven 2.1GB drives (5 data, 1 parity, 1 hot swap disk) - which means we have 14 2.1 GB disks for 20GB of total usable storage with 4.2GB tied up for striping and 4.2GB of swapable (a fairly decent ratio IMO)

        As for the performance issue - it all depends on how much writing vs reading your site will be doing as well as when the writing will be done. (Or site does roughly 10% writes 90% reads)         

        To counter your complaint about adding drives to the RAID - you really shouldn't need to add drives if you plan ahead.

        Max your array out with the largest number of disks and the largest capacity disks. (At the time, 2.1GB half-height drives were the largest in that size.)

        If you do need to add storage capacity to the system. You have two options: 1) export the database, add disks to the array (if you didn't

		fill the array in the first place like I suggested above)
		or replace every disk in the array with a larger disk
		(ie replace 2.1GB drives with 4GB drives)
		reinitialize the array and partitions
		then import the database.
		(Actually, if you kept the same partition sizes, you may
		(be able to use a cold backup instead of an import/export)

	2) Start off in the beginning with all RAID arrays maxed out,
	Then when you need more disk storage space, add another array
	with the maximum number of disks.

	A third option would be to try and keep your database from

growing larger that your diskspace - but obviously in some situations this is very difficult.

Just my $0.02.

-Al

-- 
Al Dorrington                                      
awdorrin_at_ictest.delcoelect.com                     Database Admin
Delco Electronics - IC CIM                         Unix  Sysadmin
Kokomo, Indiana, USA                               Phone: 317.451.9655

Received on Wed Jan 31 1996 - 00:00:00 CET