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I'm going to provision a server with 2 raid controllers, both controllers have the same specs but the cache, being one a 2GB cache and the other a 4GB cache controller, let call them respectively SC (small cache) and BC (big cache).

I also have 2 sets of disks: a 4-disks set with 12gbs 15k SAS disks to be setup in raid 10 and a 4 set 6gbs SSD to be set up in raid 5 (expand space at the cost of a more unreliable raid level).

db transactions will occur on ssd, while documents and mails will be stored in SAS.

My initial idea was to give more cache to SSD, but honestly I don't know if this is really useful. namely I don't really know if RAID write cache is useful at all with databases on SSD.

would like to have any insight on which cache reserve for which disk, avoiding to waste cache where not needed/useful.

thank you.

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Databases generally perform much better on top of a RAID10 rather than RAID5/6, so I really suggest you to reconsider your storage setup. I would setup the system as below:

  • the higher performance controller (the one with 4 GB of writeback cache, probably) should be attached to the SSDs. As many RAID controllers automatically disable any local, private disk's cache, if (and only if) your SSDs are themselves powerloss-protected, you should really re-enable their internal DRAM cache. For SSDs, their private DRAM cache is key to reach high performance
  • the other controller can be attached to the spinning disks.

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