[CentOS] Cnetos 5.4 ext3 question...

Ross Walker rswwalker at gmail.com
Mon Dec 28 18:22:02 UTC 2009


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> 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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