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From: Yves Vindevogel <[EMAIL PROTECTED]>
Date: Sat 18 Jun 2005 18:18:53 CEST
To: PFC <[EMAIL PROTECTED]>
Subject: Re: [PERFORM] Multiple disks: RAID 5 or PG Cluster

There's a basic difference between striping (raid 0) and mirroring (raid 1)

With striping, each file is distributed over several disks, making the physical write faster because several disks can do the work. Same for reading, multiple disks return a part of the file.
Striping can not be used for safety/backup, if one disk fails, your file is lost (if it is partly on that failing disk). With mirroring you do not lose any disk space.

Mirroring is a technique for avoiding disasters when you have a disk failure. Every file is written twice, each time to a different disk, which is a mirror of the first one.
You effectively lose half of your diskspace to that mirror. But when a disk fails, you don't lose anything, since you can rely on the other mirrored disk.

Raid 10, which is the combination of that, has both. You have multiple disks that form your first part of the raid and you have an equal amount of disks for the mirror.
On each part of the mirror, striping is used to spread the files like in a raid 0. This is a very costly operation. You need a minimum of 4 disks, and you lose 50% of your capacity.

BTW: mirroring is always slower than striping.

On 18 Jun 2005, at 18:00, PFC wrote:

I do not know what clustering would do for you. But striping will provide a
high level of assurance that each of your hard drives will process equivalent
amounts of IO operations.

I don't know what I'm talking about, but wouldn't mirorring be faster than striping for random reads like you often get on a database ? (ie. the reads can be dispatched to any disk) ? (or course, not for writes, but if you won't use fsync, random writes should be reduced no ?)




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Met vriendelijke groeten,
Bien à vous,
Kind regards,

Yves Vindevogel
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Met vriendelijke groeten,
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Mail: [EMAIL PROTECTED] - Mobile: +32 (478) 80 82 91

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