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

[Ross Walker]

On Jan 27, 2010, at 10:20 AM, Noob Centos Admin  
<centos.admin at gmail.com> wrote:

> Hi,
>
> On 1/27/10, Ross Walker <rswwalker at gmail.com> wrote:
>>
>> But if your doing mysql on top of LVM your basically doing the same,
>> cause LVM (other then current kernels) doesn't support barriers.
>>
>> Still if you have a battery backed write-caching controller that
>> negates the fsync risk, LVM or not, mysql or postgresql.
>
> This is a bit of a surpise. Am I understanding correctly that running
> postgresql or mysql on top of LVM negates any data reliability
> measures the DBMS might have in the event of an unexpected shutdown?
>
> I have several servers configured to run LVM on top of MD1 for the
> convenience of being able to add more space to a volume in the future.
> I didn't realize this was a reliability risk. :(

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.

-Ross