[CentOS] connection speeds between nodes

John Hodrien J.H.Hodrien at leeds.ac.uk
Wed Mar 9 13:49:14 UTC 2011

On Wed, 9 Mar 2011, Ross Walker wrote:

> On Mar 8, 2011, at 12:25 PM, John Hodrien <J.H.Hodrien at leeds.ac.uk> wrote:
>> I think you're right that this is how it should work, I'm just not entirely
>> sure that's actually generally the case (whether that's because typical
>> applications try to do sync writes or if it's for other reasons, I don't
>> know).
> As always YMMV, but on the whole it's how it works.
> ESX is an exception, it does O_FSYNC on each write cause it needs to know
> for certain that each completed.

But what I was saying is, most applications benefit from async writes, which
suggests most applications do noticeably amounts of sync writes.  It doesn't
overly matter if they *can* choose to write async if hardly any of them do.

>> sync;time (dd if=/dev/zero of=testfile bs=1M count=10000;sync)
>> async: 78.8MB/sec
>>  sync: 65.4MB/sec
>> That seems like a big enough performance hit to me to at least consider the
>> merits of running async.
> Yes, disabling the safety feature will make it run faster. Just as disabling
> the safety on a gun will make it faster in a draw.

And if you're happy with that (in the case of this render farm, a fault at the
NFS level is non-fatal) then there's no problem.  I don't have a safety on my
water pistol.

>> That said, running dd with oflag=direct appears to bring the performance up to
>> async levels:
>> oflag=direct with  sync nfs export: 81.5 MB/s
>> oflag=direct with async nfs export: 87.4 MB/s
>> But if you've not got control over how your application writes out to disk,
>> that's no help.
> Most apps unfortunately don't allow one to configure how it handles io
> reads/writes, so you're stuck with how it behaves.
> A good sized battery backed write-back cache will often negate the O_FSYNC
> penalty.

All those figures *were* with a 256Mbyte battery backed write-back cache.
It's really not hard to make those figures look a whole lot more skewed in
favour of async...


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