Re: Re Raid 5+

Yechial,

> You have 12 disks. In raid 0+1 you use striping across 6 volumes.
> In raid 5 you strip across 11 disks, so you get almost double the work

They didn't tell you everything they know about RAID 0+1.

The disks may be striped into 6 logical disks, and they are written to as 6 logical disks, but when reading, all 12 disks are read from independentantly.

So a RAID 5 is not actually doing more work a READ.

But during a write, it's doing quite a bit more than RAID 0+1.

> without returning and moving the r/w head on the same disk.

Maybe if there is one user on the system, doing 1 read. Multiuser IO systems don't serialize the IO, at least not to the extent that the single user scenario requires. This is much like reorging a table so that all data is in a 'contiguous' space. It's an illusion, it doesn't really work that way.

Jared

On Sunday 03 November 2002 08:03, Yechiel Adar wrote:
> Hello Ian
>
> I heard a lecture on raid 5 disks a few weeks ago.
> The rational behind read 5 being faster then raid 0+1 is this:
> You have 12 disks. In raid 0+1 you use striping across 6 volumes.
> In raid 5 you strip across 11 disks, so you get almost double the work
> without returning and moving the r/w head on the same disk.
>
> Yechiel Adar
> Mehish
> ----- Original Message -----
> From: MacGregor, Ian A.
> To: Multiple recipients of list ORACLE-L
> Sent: Friday, November 01, 2002 2:48 AM
> Subject: RE: Re Raid 5+
>
>
> I cannot fathom Raid 5 being faster than Raid 1 tor writes. The real
> question is, is it fast enough for your users. We happen to have a 650
> terabyte database here. Even using Raid 5 disk storage would be
> prohibitedly expensive. So we use a home-built hierarchal storage system
> and store much of the data on Redwood tape drives. Users know that
> requesting data from the Redwood drives will take some time. But they
> were told to expect that. (The database is Objectivity not Oracle, and I
> have nothing to do with it). The online data as opposed to the near-line
> data is stored in Raid 5 arrays.
>
> What I don't know is what percentage of Oracle databases can run fine on
> Raid 5 vs. Raid 1. It would not surprise me if the answer was well over
> 50%
>
> Ian MacGregor
> Stanford Linear Accelerator Center
> ian_at_SLAC.Stanford.edu
> -----Original Message-----
> From: John Hallas [mailto:john.hallas_at_hcresources.co.uk]
> Sent: Thursday, October 31, 2002 12:34 AM
> To: Multiple recipients of list ORACLE-L
> Subject: Re Raid 5+
>
>
> Jared,
>
>
>
> We are certainly going to be performing extensive testing to ensure
> performance of our applications under Raid5+ is acceptable.
>
>
>
> That means it is as good if not better than that experienced under
> Raid1
>
>
>
> As I see it Oracle gain no benefit for stating that Raid5 should be
> used if they did not believe that to be the case. If there was any doubt it
> would be easier fro them to leave things as they were
>
>
>
>
>
> John

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