Raid 0 slows Reads?

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Postposted on Fri Feb 08, 2002 11:45 am

Just saw a TV show that said that Raid 0 actually slows HD "reads", but it does dramatically improve "writes". Can anyone confirm this? Is there an online article on the topic?

Thanks
Xenolith
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Postposted on Fri Feb 08, 2002 11:56 am

http://serverwatch.internet.com/article ... aid_b.html
http://www.arstechnica.com/paedia/r/raid-1.html

<font size=-1>[ This Message was edited by: Bruce on 2002-02-08 10:57 ]</font>
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Postposted on Fri Feb 08, 2002 12:04 pm

I have one word for you: BS.

Ok so thats actually two words, but you get the point. The way RAID 0 works is by splitting the data across the two (or more) drives. Block 0 goes on drive 0, block 1 - drive 1, block 2, drive 0, etc. This means that if I write a chunk of data that is 10 blocks long, I write the first block and it goes to say drive 1. Rather that wait for drive 1 to finish before I write the second block, I can jump over to drive 0 and write the second block immediately. Once drive on has finished writing, I move back there and write the third block and so forth.
The same thing applies for reading. I split the block reads across the drives and they return data in parallel. Put it together in the correct order and I get approx twice the transfer speed for two drives. For matched drives, the speedup is pretty linear till you fill the bus bandwidth. Three drives are 3x faster than one. Four drives are 4x faster than one. You lose some in overhead so you never can reach the theoretical max but you can get close.
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Postposted on Fri Feb 08, 2002 3:18 pm

On 2002-02-08 11:04, SecretSquirrel wrote:
I have one word for you: BS.

.........


I'm with you, seems strange to me. I found the show notes from the show I watched (Screensavers on TechTV), and here is what he said...
.......
Skip the RAID. RAID arrays are great for writing data to disk, but you actually take a performance hit when a RAID has to read data. You're better off with a large, single hard drive.
.........


Unless I get documentation that say otherwise, I'm not going to beleive this guy.
Xenolith
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Postposted on Fri Feb 08, 2002 3:29 pm

Scott Wasson - "Damage"
Editor - The Tech Report
Damage
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Postposted on Fri Feb 08, 2002 3:36 pm

On 2002-02-08 14:29, Damage wrote:
Ok. Let's settle this:

http://www.tech-report.com/reviews/2001 ... dex.x?pg=4

:smile:

Ummm, does this test differentiate between reading and writing. Don't see it.
Xenolith
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Postposted on Sat Feb 09, 2002 10:21 pm

OK,
I saw this show as well, and although Patrick Norton is usually dead-on, this time he was off-base. He had read some stuffs in a mag, and mis-interpeted it. In theory, if you choose a huge block size (say 256KB) for your RAID, and all your data is in extremely small chunks, (say 1KB) your reads could take a hit. The odds are tho, even if you choose a block size that is not even close to optimal to the data that you store, you should still see some improvement in reads. Now writes...that's more complicated.
On the other hand, when Patrick answered that call, it was to some 9-year-old wannabe 1337 h4><0r, so its probally best that he told the kid not to do it, even if it was for the wrong reason. Don't want the kid wipin out the family PC cause he heard on TV it was OK to do so, ya know?
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Postposted on Sat Feb 09, 2002 11:34 pm

Thanks, makes sense.

I run a game server with 3 games going at once. Games servers primarily read (during map switches and start up), not much writing except for logs, etc. That's why what Patrick said got my attention.

Some of the boys who help run the game server want to go RAID 0. I'm trying to see if it would be of great benefit. Even with the increased performance... I'm thinking game servers don't tap into the hard drive often enough to make a significant difference.
Xenolith
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Postposted on Wed Feb 13, 2002 3:12 pm

Here you go, an extensive review of a high end ATA raid card with tests using both reads and writes:
http://www.storagereview.com/articles/2 ... 450_1.html
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Postposted on Wed Feb 13, 2002 6:05 pm

There's been lots of tests lately showing read speed falls almost the same percentage as write speed increases. Even Maximum PC had an article on it. IDE RAID is more about marketing than muscle.
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Postposted on Wed Feb 13, 2002 6:39 pm

On 2002-02-13 17:05, VooBass wrote:
There's been lots of tests lately showing read speed falls almost the same percentage as write speed increases. Even Maximum PC had an article on it. IDE RAID is more about marketing than muscle.

Is there an online version of that Max PC article? I can't seem to find it. Thanks.
Xenolith
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Postposted on Wed Feb 13, 2002 7:18 pm

Wouldn't your server be better off running a RAID 1 config? I would have thought that this was a question of reliability over speed... and give us some figures on the amount of read/write that your server does... I dunno how you would measure it, but I'm sure you could find a way :smile:

And as we start to talk about servers I start to go out of my depth and drown horribly :smile:,
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