On Wed, 2006-04-12 at 13:10, Ted Byers wrote: > > ----- Original Message ----- > > From: "Merlin Moncure" <mmoncure@xxxxxxxxx> > > To: "Janning Vygen" <vygen@xxxxxx> > > Cc: <pgsql-general@xxxxxxxxxxxxxx> > > Sent: Wednesday, April 12, 2006 12:31 PM > > Subject: Re: [GENERAL] Hardware related question: 3ware 9500S > > [snip] > > > > - I want to know if 3ware 9500 S is recommended or if its one of those > > > controllers which sucks. > > > > escalade is a fairly full featured raid controller for the price. > > consider it the ford taurus of raid controllers, it's functional and > > practical but not sexy. Their S line is not native sata but operates > > over a pata->sata bridge. Stay away from raid 5. > > > Hi Merlin > > Why? What's wrong with raid 5? I could well be wrong (given how little > attention I have paid to hardware over the past few years because of a focus > on developing software), but I was under the impression that of the raid > options available, raid 5 with hot swappable drives provided good data > protection and performance at a reasonably low cost. Is the problem with > the concept of raid 5, or the common implementations? > > Do you have a recommendation regarding whether the raid array is built into > the server running the RDBMS (in our case PostgreSQL), or located in a > network appliance dedicated to storing the data managed by the RDBMS? If > you were asked to design a subnet that provides the best possible > performance and protection of the data, but without gold-plating anything, > what would you do? How much redundancy would you build in, and at what > granularity? There have been NUMEROUS discussions of RAID-5 versus RAID 1+0 in the perform group in the last year or two. Short version: RAID 5 is useful, with large numbers of drives, for OLAP type databases, where you're trying to get as much storage as possible from your drives. RAID 5 pretty much REQUIRES battery backed cache for decent write performance, and even then, will saturate faster than RAID 1+0. RAID-5 cannot survive multiple simultaneous drive failures. RAID 1+0 requires better than average controllers, since many serialize and lockstep data through the various layers of RAID on them. It provides less storage for a given number of drives. It is faster for OLTP workloads than RAID-5. RAID 1+0 can survive multiple drive failures as long as two drives in the same mirror set do not fail at once. With increasing number of drives, the chances of a RAID 5 failing go up linearly, while the chances of RAID 1+0 failing due to multiple drive failure stay the same.
