Neil Brown wrote:
Sorry, for some reason I read that as being in stripes instead of bytes, which would make it 128k for size only 2. My misread.On Friday December 9, tmrbill@xxxxxxx wrote:On Fri, 9 Dec 2005, Neil Brown wrote:On Friday December 9, kylewong@xxxxxxxxxx wrote:Hi, I found that there's a new sysfs "stripe_cache_size" variable. I want to know how does it affect RAID5 read / write performance (if any) ? Please cc to me if possible, thanks.Would you like to try it out and see? Any value from about 10 to a few thousand should be perfectly safe, though very large values may cause the system to run short of memory. The memory used is approximately stripe_cache_size * 4K * number-of-drivesWhat??? I hope that's a typo... 1 - there's no use of the sysfs variable?'stripe_cache_size' is the sysfs variable. Yes, it is used.2 - that's going to be huge, 128k * 4k * 10 = 5.1GB !!!That is why I warned to limit it to a few thousand (128k is more than a few thousand!).
I just ran bonnie over a 5drive raid5 with stripe_cache_size varying in from 256 to 4096 in a exponential sequence. (Numbers below 256 cause problems - I'll fix that). Results: 256 cage,8G,42594,93,151807,38,50660,18,38610,91,172056,38,912.8,2,16,4356,99,+++++,+++,+++++,+++,4389,99,+++++,+++,14091,100 512 cage,8G,42145,92,186535,44,60659,21,42249,96,172057,37,971.9,2,16,4407,99,+++++,+++,+++++,+++,4452,99,+++++,+++,13909,99 1024 cage,8G,42250,92,210407,50,61254,21,42106,96,172575,37,903.1,2,16,4370,99,+++++,+++,+++++,+++,4395,99,+++++,+++,13809,100 2048 cage,8G,42458,92,229577,55,61762,21,41965,96,168950,36,837.9,2,16,4373,99,+++++,+++,+++++,+++,4460,99,+++++,+++,14084,100 4096 cage,8G,42305,92,250318,62,62192,21,42156,96,170692,38,981.8,3,16,4380,99,+++++,+++,+++++,+++,4426,99,+++++,+++,13723,99 Seq Write speed ^ Increases substantially. Seq Read ^ Doesn't vary much. Seq rewrite ^ improves a bit So for that limited test, write speed is helped a lot, read speed isn't. Maybe I should try iozone... NeilBrown
-- bill davidsen <davidsen@xxxxxxx> CTO TMR Associates, Inc Doing interesting things with small computers since 1979 - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
