Re: [RFC] extending splice for copy offloading

[Ric Wheeler <rwheeler@redhat.com>]

On 09/27/2013 12:47 AM, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 11:23 PM, Ric Wheeler <rwheeler@redhat.com> wrote:
>> On 09/26/2013 03:53 PM, Miklos Szeredi wrote:
>>> On Thu, Sep 26, 2013 at 9:06 PM, Zach Brown <zab@redhat.com> wrote:
>>>
>>>>> But I'm not sure it's worth the effort; 99% of the use of this
>>>>> interface will be copying whole files.  And for that perhaps we need a
>>>>> different API, one which has been discussed some time ago:
>>>>> asynchronous copyfile() returns immediately with a pollable event
>>>>> descriptor indicating copy progress, and some way to cancel the copy.
>>>>> And that can internally rely on ->direct_splice(), with appropriate
>>>>> algorithms for determine the optimal  chunk size.
>>>> And perhaps we don't.  Perhaps we can provide this much simpler
>>>> data-plane interface that works well enough for most everyone and can
>>>> avoid going down the async rat hole, yet again.
>>> I think either buffering or async is needed to get good perforrmace
>>> without too much complexity in the app (which is not good).  Buffering
>>> works quite well for regular I/O, so maybe its the way to go here as
>>> well.
>>>
>>> Thanks,
>>> Miklos
>>>
>> Buffering  misses the whole point of the copy offload - the idea is *not* to
>> read or write the actual data in the most interesting cases which offload
>> the operation to a smart target device or file system.
> I meant buffering the COPY, not the data.  Doing the COPY
> synchronously will always incur a performance penalty, the amount
> depending on the latency, which can be significant with networking.
>
> We think of write(2) as a synchronous interface, because that's the
> appearance we get from all that hard work the page cache and delayed
> writeback code does to make an asynchronous operation look as if it
> was synchronous.  So from a userspace API perspective a sync interface
> is nice, but inside we almost always have async interfaces to do the
> actual work.
>
> Thanks,
> Miklos

I think that you are an order of magnitude off here in thinking about the scale 
of the operations.

An enabled, synchronize copy offload to an array (or one that turns into a 
reflink locally) is effectively the cost of the call itself. Let's say no slower 
than one IO to a S-ATA disk (10ms?) as a pessimistic guess. Realistically, that 
call is much faster than that worst case number.

Copying any substantial amount of data - like the target workload of VM images 
or media files - would be hundreds of MB's per copy and that would take seconds 
or minutes.

We should really work on getting the basic mechanism working and robust without 
any complications, then we can look at real, measured performance and see if 
there is any justification for adding complexity.

thanks!

Ric

>