Dear Stan, In message <52C5A9AA.9090300@xxxxxxxxxxxxxxxxx> you wrote: > > > filesystem blocks and memory pages aren't necessarily 4K, though that > > is the most common size. > > Yes, they are necessarily 4K in Linux. Linux only supports page sized > BIO for consistency across the memory manager and IO subsystems. Most > architectures which Linux currently supports have hardware page sizes > greater than 4K, for instance IA64 supports 4k/8k/16k, even a 4GB page > size. But it was decided long ago to stick with 4K for a number of > reasons, one of these is stated above. For background on this Google is > your friend. Well, you can tune the page size - and if you need no file system support (like when implementing a RAID controller card) making the page size exactly the same as the chunk size will allow for some nice performance optimizations (as you can avoid a lot of large memcpy() operations). We did this (some 6 years ago) for the (then AMCC) PPC440SPe processors; I can't find the old document any longer on APM's web site, but there is still a copy here ([1]) which shows the effect. So yes, tuning the system page size can have considerable impact, but only for special-purpose applications, and when optimising for large sequential I/O (which appears to be how the RAID controller manufacturers are testing / optimizing their systems). Fact is, with a file system on top of the RAID array, and with our typical work load of many very small files, A RAID6 with chunk size 16k will give much better results that with chunk size 64k - and anything even bigger will be worse. [1] ftp://ftp.denx.de/pub/demos/RAID-demo/doc/RAIDinLinux_PB_0529a.pdf Best regards, Wolfgang Denk -- DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd@xxxxxxx "The first rule of magic is simple. Don't waste your time waving your hands and hoping when a rock or a club will do." - McCloctnik the Lucid -- 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
