On Jan 28, 2010, at 7:27 PM, Christopher Chan <christopher.chan at bradbury.edu.hk > wrote: > On Thursday, January 28, 2010 10:48 PM, Ross Walker wrote: >> >> On Jan 27, 2010, at 7:50 PM, Christopher Chan<christopher.chan at bradbury.edu.hk >>> wrote: >> >>> >>>> Sorry to be the bearer of bad news, but on top of LVM on CentOS/ >>>> RHEL >>>> the best assurance your going to get is fsync(), meaning the data >>>> is >>>> out of the kernel, but probably still on disk write cache. Make >>>> sure >>>> you have a good UPS setup, so the disks can flush after main power >>>> loss. >>> >>> Or turn off write caching... >> >> Have you tried doing any kind of write with write caching turned off? >> It is so horribly slow to make it almost useless. > > If they needed the performance in the first place, I doubt they > would be > using md raid1. You want performance and reliability? Hardware raid + > bbu cache. Otherwise, it is turn off write caching unless the i/o path > supports barriers. Yes, but a lot of people jumped on the SW RAID is just as good or better then HW RAID bandwagon and well there is no battery backed up write cache then. >> >> If you need to turn write-caching off then I would start looking at >> SSD drives with capacitor based caches. >> > > How do those compare with bbu nvram cards for external data + metadata > journaling? Slightly slower then nvram, but don't suffer from the write-cache filling up under load. -Ross