Re: What is the point of RAID?
On Fri, 7 Nov 2008 23:29:55 -0500
"Douglas A. Tutty" <firstname.lastname@example.org> wrote:
On Fri, Nov 07, 2008 at 10:20:51PM -0600, Mark Allums wrote:
If you do only have three drives, add it to the raid1 array.
Hm, if you do that, is there any other use for the third disk than as a
I think my name did not add appreciably to the subject, here, but, to
answer the question, I didn't understand what Douglas meant. In a RAID
0, (striping) you can stripe across any number of drives. In RAID 1
(mirroring) you can mirror one drive any number of times. I am not sure
what the point would be of the latter.
Now, often, one can "migrate" an array from one type to another, e.g.
from RAID 0 to RAID 5. Perhaps he meant, if you have three drives,
add the fourth to it.
Douglas, could you clarify?
My main concern originally was that a three-drive RAID 5, done in
software, gets slower and slower as the logical drive fills up. So,
your idea of getting a third and doing RAID 5 would not be my first
choice. With more drives you gain in some areas, and lose in others.
RAIDs 3,4,5,6 and the variants like 5EE and 50 really do best with a
hardware RAID card with a fast co-processor and lots of cache memory.
RAID 10 is doing a RAID 1 mirror across two pairs of drives, call them
pair A and pair B. Then, you do a RAID 0 stripe across drive A and B,
to form the final logical drive. With RAID 10, you can lose two drives
at once in three ways, and in two of the ways, you still have all your
data. The third is when you lose both drives of a pair at the same
time. Then it is just like losing a drive in a RAID 0.
(A RAID 0+1 is striping pairs, and mirroring the stripes. You lose two
drives simultaneously in one of three ways, but *two* of the three, you
RAID 0 is fast. RAID 1 is redundant and kind of fast. RAID 10 is
nearly as fast as RAID 0, and it has the redundancy. Losing a single
drive is no big deal. Add a spare, and rebuild.
RAID 5 allows losing a single drive to be no big deal. Also, when done
right, with the right number of drives and a good RAID card, is has
performance like RAID 0, or even better. It uses striping combined with
RAID 6 is similar to 5, except that it allows the loss of two drives.
RAID 2 is not generally considered satisfactory, and is not done
anymore. 3 and 4 are generally legacy modes. New arrays are seldom, if
ever, done as 3 or 4.
For the desktop machine, 0, 1, 10, and 5 are feasible. All can be done
in software. 0, 1, and 10 have reasonable performance done in software.
5 starts out with good performance, but as the drive fills up, the
performance drops. One reason is that you are writing more data than
you think. The parity must be calculated and written. It increases the
amount of data to be written by fifty percent. Also, reads must include
the parity data, and then the parity must be checked. RAID 5 in
software is not good for a video editing system, or any system with
combined high CPU and high data throughput, like perhaps a server with
several virtual servers running database apps.
RAID 5 is popular in some desktops because it seems advanced to the
user, but only requires three drives and a device driver to implement,
and can generally be booted from.
However, as has been pointed out, you add a third drive to two existing
ones, and you don't get any more storage capacity. It just sort of gets
"soaked up". Three drives are three times as likely to have a failure
as one. (Four, in RAID 10, has a similar down side.) I would stick
with RAID 1, for being the most cost effective for a desktop.
And, I must repeat: Do regular backups of anything you'd hate to lose,
AND TAKE IT OFF SITE!