(Note: most of this is kuwento, especially for those planning to get a
3Ware, I have a question on RAID 10 versus RAID 5 in the third paragraph,
though).
(Warning: long post, as usual)
Hi there everyone,
This may be a little off-topic, but perhaps a number of you will be
interested. My father just arrived this morning (early this morning, see
timestamp) from Hong Kong. Aside from four lovely books (hehehe), he got
four 30.1GB IBM hard drives and a 3Ware Escalade 6400 controller. I'm
reading the manual (something I don't normally do, but the controller came
before I could piece together the rest of the server so I can't plunge in
anyway, and I'm excited) and it's a very interesting thin piece of printed
material.
For one the darned thing is honest and competent when it says it supports
"RAID 10". It even has diagrams to support its text explanations of the
various RAID levels that rocks the pants off what I read online. This card
supports RAID 0, 1, 10 and 5 (except on Windows 98/ME, where RAID5 is not
yet supported, bwahahaha!!!), plus JBOD.
This brings me to my question: RAID 10 or RAID 5? I haven't found anything
comparing the two as far as performance is concerned. Here's a little
kuwento about the two RAID levels for those who don't know, though. It can
also help in figuring out where I'm coming from (why I'm trying to decide
between the two).
RAID 10 as the manual describes: "The first two drives are mirrored as a
fault tolerant array using RAID 1. The third and fourth drives are
mirrored as a fault tolerant array using RAID 1. The two mirrored arrays
are then grouped as a striped RAID 0 array using a two tier structure."
With this you get the total number of drives (assumed to be an even
number) divided by two in terms of capacity.
RAID 5 is like RAID 4 except that the parity information is spread out
across the disks. Capacity = [(size of smallest drive) * (n - 1)], where n
is the number of drives in the array. Storage efficiency = [(n - 1) / n].
My deduction as far as handling failures are concerned is that RAID 10 can
handle up to two failed drives at the same time, given that they don't
belong to the same mirrored array. RAID 5 seems to only be able to handle
one at a time. However, RAID 5 gives me one disk's worth (since my n = 4)
more of capacity (although I honestly do not know how I can fill that as
of now). Anyone care to comment on this?
Also, there is the issue of the stripe size, for RAID 0 and RAID 10 the
controller supports a stripe size of 64KB, 120KB, 250KB 512KB, or 1mb (I
wonder why they use that kind of capitalization). RAID 5 only does 64KB.
What's the rule of thumb as far as selecting a stripe size is concerned?
Also, would anyone have an explanation or something as far as the
performances of RAID 10 and RAID 5 are compared to each other given
otherwise identical setups?
The 3Ware controllers are interesting for Linux users and administrators
because support for the 3Ware has been built into the kernel since early
2.2, I think. For sure the latest 2.2 and 2.4 support them. The
installation kit also includes modules pre-built for RedHat and SuSE, and
the source code for building the modules (although I'm planning to go
static kernel support).
I have yet to find out what's so RedHat-ey and SuSE-y about the 3DM
installation, because I intend to use Debian <wink>.
The package looks very interesting, though. And you can be sure to hear
from me about 3Ware when I actually set it up together with a server I'm
building. :)
--> Jijo
---
Linux, MS-DOS, and Windows NT ...
... also known as the Good, the Bad, and the Ugly
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