On Jan 28, 2010, at 7:27 PM, Christopher Chan <christopher.chan@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@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