[CentOS] Cnetos 5.4 ext3 question...
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.
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