Fwd: Multiple disks: RAID 5 or PG Cluster

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> From: Yves Vindevogel <yves(dot)vindevogel(at)implements(dot)be>
> Date: Sat 18 Jun 2005 18:18:53 CEST
> To: PFC <lists(at)boutiquenumerique(dot)com>
> 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
> Implements
>