Is SCSI asynchronous IO really slower than synchronous one?

Question

I am completely confused with what I had just read on these two pages:

http://www.scsifaq.org/scsifaq.html#_Hlk410546176 http://www.cse.scu.edu/~tschwarz/coen180/LN/scsi.html

Both claim that SCSI asynchronous IO is slower than synchronous one. How come? Why would anyone bother to write async procedure only to loose performance?

It contradicts my expectation!

Especially that, in case of SSD, I am using libaio to read asynchronously to get much better performance.

Furthermore, the first list synchronous as send send send wait wait wait... That is exactly what asynchronous is in case of libaio.

Anyone can shed a light on it for me, please?!

Edit 1

Let me clarify my problem. I may have a problem with correct understanding of a difference between asynchronous and synchronous IO.

• Synchronous: It waits for each operation to complete
• Asynchronous: It never waits for each operation to complete

Now, I would assume that if I had to read sequentially a few gigs of data, reading it in a simple synchronous loop:

while ( num_blocks-- > 0 )
    read_block() ;

Would have a "request, pause, request" type of queries. Practically I see that if I change it to multithreaded procedure, span several (6-8) threads that would take 'next location to read' and then requested it, I could get almost maximum read performance.

Something like this:

struct request
{
    ...

    boost::mutex mutex ;
    uint64_t     block ;

    uint64_t get_block_location()
    {
        boost::mutex::scoped_lock lock( mutex ) ;
        return block++ ;
    }
} ;

void readth( request* r )
{
    try
    {
        for( ;; )
        {
            read_block( r->get_block_location()) ;
        }
    }
    catch( const FinishedException& )
    {}
}

I thought, and now, I think I start getting it after rethinking what I had just read with a help from @Robert Harvey that async/sync is really only a matter of 'saving time' on waiting for the result...

I am leaving this question just in case someone has cognitive problems like mine!

Answer 1

Asynchronous I/O is not about improving the speed of disk performance. You can't do that by making I/O asynchronous; the disk platters spin at the same speed regardless of the manner in which you make I/O calls.

The purpose of Asynchronous I/O is to make it possible to go and do other things while you're waiting on the data. While that can appear to be a performance improvement, what you're really doing is freeing up a processor from having to idle while the data arrives.