[CentOS] Centos/Linux Disk Caching, might be OT in some ways

Christopher Chan christopher.chan at bradbury.edu.hk
Fri Jan 29 00:27:39 UTC 2010

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.

> 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 

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