On Tue, Oct 20, 2015 at 7:30 AM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Tue, Oct 20, 2015 at 3:14 AM, Birta Levente <blevi.linux@xxxxxxxxx> wrote: >> Hi >> >> I have a supermicro SYS-1028R-MCTR, LSI3108 integrated with SuperCap module >> (BTR-TFM8G-LSICVM02) >> - 2x300GB 10k spin drive, as raid 1 (OS) >> - 2x300GB 15k spin drive, as raid 1 (for xlog) >> - 2x200GB Intel DC S3710 SSD (for DB), as raid 1 >> >> So how is better for the SSDs: mdraid or controller's raid? > > I personally always prefer mdraid if given a choice, especially when > you have a dedicated boot drive. It's better in DR scenarios and for > hardware migrations. Personally I find dedicated RAID controllers to > be baroque. Flash SSDs (at least the good ones) are basically big > RAID 0s with their own dedicated cache, supercap, and controller > optimized to the underlying storage peculiarities. > >> What's the difference between Write Back and Always Write Back with supercap >> module? > > No clue. With spinning drives simple performance tests would make the > caching behavior obvious but with SSD that's not always the case. I'm > guessing(!) 'Always Write Back' allows the controller to buffer writes > beyond what the devices do. We're running LSI MegaRAIDs at work with 10 SSD RAID-5 arrays, and we can get ~5k to 7k tps on a -s 10000 pgbench with the write cache on. When we turn the write cache off, we get 15k to 20k tps. This is on a 120GB pgbench db that fits in memory, so it's all writes. Final answer: test it for yourself, you won't know until you do which is faster. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
