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Re: Raid 1 or Raid 0/1

From: Art S. Kagel <kagel_at_bloomberg.net>
Date: Tue, 02 Mar 1999 09:51:45 -0500
Message-ID: <36DBFB01.2BFB@bloomberg.net>


mark.powell_at_eds.com wrote:
>
> In article <7beao8$hsn$1_at_news.netway.at>,
> "Richard Rotter" <richard_rotter_at_mediaprint.at> wrote:
> > Hello everyone!
> >
> > I have the following problem:
> >
> > We change the UNIX-DB-Server from HP (HPUX 10.20 EMC Raid 1) to SUN Solaris
> > 2.6 with Fibre-Channel-Architektur.
> >
> > My Problem is to make RAID 1 or RAID 0/1 on the new architektur? Our
> > Application is a Application with 90 pct random-access! Is it a good idea to
> > make RAID 0/1 (Stripes) or make only Raid 1??????????????????
> >
> > reguards
> > Richy
> >
>
> I like RAID 0 + 1 which I believe is OS level stripping with full mirroring.
> The mirrors protect you from loss of a disk device while the OS level
> stripping help prevent disk I/O bottlenecks. I much prefer RAID 0+1 over
> RAID 5 for performance reasons.

Let me just add: Let's keep our terminology clear. One must be careful, there is what is called RAID 0+1 and also RAID 1+0 (or RAID10).
The handle RAID 0+1 is normally attached to a disk array created as two stripe sets mirrored to one another. The RAID 1+0 moniker is used for a set of 'N' mirrored pairs striped together. Performance-wise and safety-wise these should be equivalent and so many do not make the distinction. HOWEVER, at recovery time there is a world of difference.

RAID 0+1 requires that the entire stripe be rebuilt from the mirror stripe set when a drive is replaced. This means that performance is degraded significantly during the rebuild process since all drives in the mirror stripe will have to be actively read to recover the down stripe. Degradation can exceed 70% of capacity if the mirror driver favors recovery over live performance or the recovery may be delayed significantly, putting your data at risk if ANY drive on the mirror fails during the extended recovery, if the mirror driver favors live performance over recovery speed.

In RAID 1+0 ONLY the one drive pair that is being recovered is degraded during recovery. Performance for the stripe is then degraded by less than 1/N of capacity. Also recovery time and exposure to the risk of a mirror drive failing during recovery is minimized since only a single drive is being recovered and the failure of any other drive except that one mirror drive is safe.

Art S. Kagel Received on Tue Mar 02 1999 - 08:51:45 CST

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