On 07/15/2014 11:18 AM, Patrick J. LoPresti wrote:
> On Tue, Jul 15, 2014 at 11:11 AM, ToddAndMargo <[log in to unmask]> wrote:
>>
>>> Who would this differ from the "sync" command?
>>
>>
>> "who" should have been "how"
>
> As someone else said, "sync" just synchronizes the copy on disk with
> the copy in memory; i.e. it forces a flush of dirty pages to disk. The
> kernel will still keep a copy of those pages cached in RAM, forever,
> or until it needs that RAM for something else.
>
> That copy in RAM can mess up your performance tests. Specifically, if
> all file meta-data (specifically file sizes and timestamps) is cached,
> rsync will not actually touch either the source or destination disk at
> all.
>
> "sysctl -w vm.drop_caches=3" forces the kernel to forget what it
> cached, setting a clean slate for an honest benchmark.
>
>   - Pat
>


Got the same results the next day after powering off the machine
for the night.  Does that help?