Re: Is Raid 5 really that bad for Oracle?

Daniel Morgan wrote:
>
> Connor McDonald wrote:
>
> > Niall Litchfield wrote:
> >
> >>"Cary Millsap" <cary.millsap .at. hotsos .dot. com> wrote in message news:<10gu902j8cmcs7b_at_corp.supernews.com>...
> >>
> >>>2. But mirroring is expensive per byte of storage. Hence RAID levels 2, 3,
> >>>4, 5, and 6 were proposed. Their design goals were to lessen the
> >>>expense-per-byte of storage of RAID level 1 (mirroring). For example, with
> >>>G=5 RAID level 5, the price of resilience per byte of storage is 5/4 of a
> >>>4-disk array instead of 8/4.
> >>
> >>I meant to make a similar point originally, but didn't. Namely that
> >>RAID5 tends to make sense from a COST/GB point of view and SAME tends
> >>to make sense from a COST/IO point of view. Often these two views are
> >>where the disagreement lies. If my sysadmins ask me how much storage I
> >>want, they do not want to be told (say) 100gb and 2500 IO/sec. They
> >>just want the former figure.
> >>
> >>Niall
> >
> >
> > True story...
> >
> > Company X moving from JBOD to SAN. JBOD totals around 800G so the SAN
> > is populated to 800G as well...Of course, in the JBOD, each 18G disk
> > ends up being around 17.5G available to the OS. In the SAN, each 36G
> > disk ends up being around 30G to the OS
> >
> > Result:
> > - They've halved the number of spindles
> > - They've not enough disk space
> > but hey...at least we've got a SAN
> >
> > :-)
>
> Good point. But is the database really writing to those spindles or is
> it, more likely, writing to a very large RAM cache rendering the loss
> irrelevant?
>
> --
> Daniel A. Morgan
> University of Washington
> damorgan_at_x.washington.edu
> (replace 'x' with 'u' to respond)

Write Cache - great

Read Cache - not so great, after all, anything that was gonna be usefully cached was probably already in the buffer cache anyway.

Sad fact (at this client):

"Too difficult" to carefully layout volumes...after all, its a "magical" SAN. Thus, everything's just thrown into a great mix and thus any special separation of disks to get certain cache benefits is gone anyway...

so sad...so sad.

-- 
Connor McDonald
Co-author: "Mastering Oracle PL/SQL - Practical Solutions"
ISBN: 1590592174

web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: connor_mcdonald_at_yahoo.com

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