[CentOS] Cnetos 5.4 ext3 question...

Tom Bishop bishoptf at gmail.com
Mon Dec 28 18:46:24 UTC 2009


Thanks for the explanation, looks like I need to go read some more about
barriers to truly understand what is going on.....

On Mon, Dec 28, 2009 at 12:22 PM, Ross Walker <rswwalker at gmail.com> wrote:

> On Dec 28, 2009, at 12:07 PM, Tom Bishop <bishoptf at gmail.com> wrote:
>
>
> On Mon, Dec 28, 2009 at 11:03 AM, < <david at pnyet.web.id>david at pnyet.web.id
> > wrote:
>
>> I'm using ext3 on my CentOS box, so far so good, I don't get any problem.
>> Sometimes my server shutdown when power is cut, but CentOS still running
>> well and nothing corruption files or anything after start again.
>
> Thanks guys for the responses, can anyone explain what the hoopla is then
> about ext4 and performance issues and barriers being enabled, there was also
> some talk about that being an potential issue with ext3?  I've tried to
> google and look but have not found a good explanation on what the issue
> is....
>
>
> Barriers expose the poor performance of cheap hard drives. They provide
> assurance that all the data leading up to the barrier and the barrier IO
> itself are committed to media. This means that the barrier does a disk flush
> first and if the drive supports FUA (forced unit access, ie bypass cache),
> then issues the IO request FUA, if the drive doesn't support FUA then it
> issues another cache flush. It's the double flush that causes the most
> impact to performance.
>
> The typical fsync() call only assures data is flushed from memory, but
> makes no assurance the drive itself has flushed it to disk which is where
> the concern lies.
>
> Currently in RHEL/CentOS the LVM (device mapper) layer doesn't know how to
> propogate barriers to the underlying devices so it filters them out, so
> barriers are only currently supported on whole drives or raw partitions.
> This is fixed in the current kernels, but has yet to be backported to RHEL
> kernels.
>
> There are a couple of ways to avoid the barrier penalty. One is to have
> nvram backed write-cache either on the contoller or as a separate
> pass-through device. The other is to use a separate log device on a SSD
> which has nvram cache, newer ones have capacitor backed cache or a
> standalone nvram drive.
>
> -Ross
>
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