Ross Walker wrote:
> On Dec 28, 2009, at 12:07 PM, Tom Bishop <[email protected]
> <mailto:[email protected]>> wrote:
>
>>
>> On Mon, Dec 28, 2009 at 11:03 AM, <
>> <mailto:[email protected]>[email protected]
>> <mailto:[email protected]>> 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.
Did linux ever get a working fsync() or does it still flush the entire
filesystem buffer?
--
Les Mikesell
[email protected]
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