> Von: linux-raid-owner@xxxxxxxxxxxxxxx [linux-raid-owner@xxxxxxxxxxxxxxx]" im Auftrag von "Matt Garman [matthew.garman@xxxxxxxxx] > Gesendet: Freitag, 31. Juli 2015 17:23 > An: Mdadm > Betreff: Thoughts on big SSD arrays? > > Every few years I reprise this topic on this mailing list[1], [2]. > Basically I'm just brainstorming what is possible on the DIY front > versus purchased solutions from a traditional "big iron" storage > vendor. Our particular use case is "ultra-high parallel sequential > read throughput". Our workload is effectively WORM: we do a small > daily incremental write, and then the rest of the time it's constant > re-reading of the data. Literally 99:1 read:write > > I continue to be inspired by the "Dirt Cheap Data Warehouse (DCDW)" > [3]. SSD are getting bigger and prices are dropping rapidly (2 TB > SSDs available now for $800). With our WORM-like workload, I believe > we can safely get away with consumer drives, as durability shouldn't > be an issue. > > So at this point I'm just putting out a feeler---has anyone out there > actually built a massive SSD array, using either Linux software raid > or hardware raid (technically off-topic for this list, though I hope > the discussion is interesting enough to let it slide). If so, how big > of an array (i.e. drives/capacity)? What was the target versus actual > performance? Any particularly challenging issues that came up? Hi Matt, maybe not 100% matching... We wanted to start a 22 HDD md raid 6 last year for one of our storage servers. Doing some tests we experienced slow random write I/Os and got discouraged. So we headed over to hardware raid. To get a clearer picture we did further analysis. The bootleneck arised from two sources. md issued only 4K I/Os and it always did a reconstruct write. Thus massive write amplification. Several patches in 4.1 mitigate the situation. In between I like a lot of things in md raid 4/5/6. Especially the parity calucation massively benefits from high power CPUs. Back to the reason for my explanation: The biggest "area under construction" in md raid 4/5/6 is the internal stripe cache handling. Allocating, flushing and freeing stripes is based on solid but not tuned algorithms (like LRU). IIRC it does not even have a scheduling or queueing. That could become your bottleneck. Maybe massive CPU power easily compensates the gap. That said. Your read mostly setup might avoid a lot of headaches but you need to push it to the limits yourself. I do not see benchmarks in the net that closely fit your case. So I advise to enable ramdisks in the Linux kernel and build a raid 6 on top of /dev/ramX. A synthetic test according to your needs should give an idea of scalability and overhead of md raid. Good luck. Markus
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