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    <p>Although it should not be necessary in most situations, you can influence the IO size used by GCD with the <code>dispatch_io_set_high_water(3)</code> and <code>dispatch_io_set_low_water(3)</code> APIs. </p> <p>GCD will not read or write chunks larger than a channel's high water mark. Read/write handlers will also never be called with a data object that is smaller than the low water mark.</p> <p>E.g. by setting the low water mark of <code>input_</code> in your example to 1MB you can ensure that your current read callback does not pass data objects smaller than 1MB to <code>dispatch_io_write(3)</code>.</p> <p>If this control doesn't suffice in your situation, you could also combine multiple data objects received from successive invocations of your read handler via <code>dispatch_data_create_concat(3)</code> until they reach a size big enough to pass to <code>dispatch_io_write(3)</code>.</p> <p>Hopefully this should not be necessary however, setting the source side's low water mark to a multiple of the preferred source chunk size big enough to reach the preferred destination chunk size and setting the destination channel's high water mark to the preferred destination chunk size (or a multiple thereof) should give you the same performance as your current NSStream-based solution.</p> <p>You can check out the specifics of the GCD IO buffer policy in the <a href="http://libdispatch.macosforge.org/trac/browser/branches/Lion/src/io.c#L1928" rel="noreferrer">implementation</a>.</p> <p>In any case, please make sure to file a <a href="http://bugreport.apple.com" rel="noreferrer">bug</a> with the specifics of any case where you see performance issues with the default GCD IO buffering.</p>
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    1. COAwesome. I've read through the implementation and have successfully been able to get the correct amount of *callbacks* by using a `dispatch_data_t` plus `dispatch_data_create_concat` and `dispatch_data_create_subrange`, but have yet to actually observe the `read`/`write` calls (I want to double check) and need to see whether playing with the watermarks gives me a better solution. Will update once I get around to do more tests.
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    2. COAlright, so the syscalls to `read` and `write` do indeed have the expected parameters when I set the high water mark to the respective preferred IO sizes, as expected. So with that I can solve my "problem". Setting the low water mark would only affect how often the blocks get called for "short" reads/write, I'd still have to accumulate the read data and schedule the partitions for writing manually. Which isn't a problem, of course.
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    3. COHowever, it turns out out the "optimized" version doesn't run any faster than my "naive" implementation, which suggests that the kernel is already smart enough and the real problem was likely wrong measurement when comparing the NSStream vs. GCD solution. Ouch. Thanks a lot for helping me, though.
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