So, are you saying raid 1 has faster read/write performance than raid 0?
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Dan Sorenson
Sent: Thursday, February 09, 2006 12:57 AM
To: [email protected]
Subject: Re: [hlds] Re: I need help!!
At 12:44 AM 2/8/2006 -0800, m0gely wrote:
>> The quickest way to
>> boost disk read speed is to run a raid-1 config, because both disks can
>> be read from alternately -- whichever one has the sector needed closest
>> to the heads gets used.
>
>Did you mean to say RAID-0? 1 is for mirroring. As for the closest
>sector remark, heh, what the heck are you talking about?
Be warned, this is going to be a little geeky. Raid-0 is no
raid at all, it's merging two or more drives into one volume. Where
data is read or written to is random. Raid-1 is mirroring. You've
two disks with the same information on all tracks and sectors. Tracks
are the outward-to-inward portions of a disk, sectors are the segments
around those disks. So let's say I have a Raid-0 with some data on it,
I might be reading all of it from disk 1 or disk 2, or maybe half
from disk 1 and half from disk 2. That's not efficient because I
have to read that data in an order, so I may read it as disk1 disk1
disk2 disk1 disk2 etc...
Raid 1 is two disks mirrored. Here's where it gets really
pretty. Let's assume they're running in synch, disk 0 and disk 1 have
10 tracks and 4 sectors per track covering 90 degrees of the platters
and are spinning together. Let's say I need to read a map from sectors
1 and 2. With Raid 1 I can read sector 1 from disk 1 and sector 2 from
disk 1 on the next revolution. That's no better than having one disk.
Let's say the disks aren't in synch. I can then read sector 1 from
disk 1 and sector 2 from disk 2 that's only 1/4 revolution behind.
Or, I can read sector 2 first from disk 1 and cache it and grab
sector 1 from disk 2 as it's coming around.
Oh, if I have to write data I have to write it to 2 disks.
So that's two write operations waiting for the disks to come
around, but again it they're out of synch I can always write to
them in the order they come around.
Raid0 means I can't predict which disk is going to be the
next one I can read from or write to. Raid-5 means I have to do
at least 2 reads or 2 writes before that data is considered valid,
which means I might have to wait for three sectors to pass by
and heads to move to the proper track before I get my data.
Raid-5 means (disk MB * (disks - 1)) is your volume size,
so 5 x 100GB disks is a 400G redundant volume. That's 80% of
disk available vs disk purchased. Raid 1 is (disk MB * disks/2)),
so you buy a gigabyte of disk and you get half that usable.
Naturally, this makes Raid-1 expensive for storage but the fastest
available. Raid-5, at 66% to over 80% capacity depending upon
the number of disks, is slower but a more efficient use of disk.
>If he had an 8 year old 1GB IDE hard drive it *wouldn't* cause the map
>changes to take 30~40 seconds.
Agreed. I was giving a lesson in isolating the limiting
factor and how best to work around it. Still, I felt the Raid-0
vs raid-1 vs raid-5 advantages and trade-offs deserved a little
more elaboration.
- Dan
* Dan Sorenson DoD #1066 A.H.M.C. #35 [EMAIL PROTECTED] *
* Vikings? There ain't no vikings here. Just us honest farmers. *
* The town was burning, the villagers were dead. They didn't need *
* those sheep anyway. That's our story and we're sticking to it. *
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