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What caught my attention was their recommendation to use RAID-6.Has the world changed? Do I need to revisit and reinvestigate this whole RAID thing (which I thought I had settled in my own mind years ago)?I don't want to be one of "those guys" who, being resistant to change and new knowledge, are forever living in a static past.Curious,DG
Well, since I probably was responsible for mentioning (among other things, like ZFS) a possibility of using Informix with RAID-6, let me try to explain why.I am somewhat familiar with the case under discussion, but maybe not with the fine points, however the primary recommendation was to use SAN with multipathing - if needed. Granted, setting up SAN would require specific expertise and extra hardware, but that would be by far most reliable and flexible solution. I do understand the attractiveness of using storage local to the system with 2 controllers and setting up mirror or RAID10 using disks attached to different controllers to remove an extra single point of failure (controller), however even in that configuration - system still remains a single point of failure.Without going into long argument about storage performance of mirror vs RAID-6, including the fact that I/O patterns for mirror are expected to be more favorable to underlying storage devices, in practical terms - with good modern controller (battery backup + reasonably sized write cache + it is enabled) most database workloads, aside from those specifically targeting I/O, will not cross threshold where difference in I/O performance between mirror and RAID-6 will be decisive factor of choosing one solution over the other.I'd go as far as to say that with current CPU speeds the optimization direction should be more in I/O avoidance whenever it's possible.If the (older) system has only 2 disks - clearly RAID-6 is not a viable option. With 6 disks and decent RAID controller - I would personally go with RAID-6, unless disks are so small that mirror can be recovered under an hour after disk failure. When one disk in mirror fails - the clock is ticking, as there is no redundancy, and despite the fact that (in theory) recovery includes only read from remaining disk and write to the new one - if for any reason remaining disk fails - the data is pretty much gone.As usual, the decision to use any type of RAID depends on specifics, but for multi-terabyte arrays I personally would feel safer with extra parity which RAID-6 provides.Now about performance. Few years ago I was evaluating IBM's FS900 flash storage with several modules which were in RAID-5. Informix was driving close to million 4k IOPS with it (sustained!) - despite all the negative things we know about this type of RAID. Can anyone make an argument - with 8 flash modules - to use mirror / RAID10 instead ? Both the IOPS and bandwidth limits at that time were from system CPU and FC adapters, not due to the fact that RAID-5 was performing worse than mirror.Even with locally attached SSDs - I am fairly sure that performance of RAID-6 will be adequate and provide more storage capacity, so that considering mirror solution would be unreasonable waste of resources.With small number of local disks capable of 150 IOPS each and write cache disabled - both mirror and RAID-6 will give awful performance with mirror being slightly less awful, but it's already 21st century - do we have to suffer needlessly?