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

Ross Walker rswwalker at gmail.com
Fri Jan 29 00:50:01 UTC 2010


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


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