Disk I/O with OpenSSH

chris rapier rapier at psc.edu
Tue Jan 30 01:51:17 EST 2007

Damien Miller wrote:
> On Fri, 26 Jan 2007, Chris Rapier wrote:
>>I was wondering if the disk I/O in OpenSSH is synchronous or 
>>asynchronous. If its not asynchronous is there any reason why doing at 
>>least asynch writes would be a bad idea? I'm just wondering if I'm over 
>>looking some obvious problem.
> What disk IO? Do you mean sftp/scp?

Yes. Mostly I'm wondering if a momentary hiccup on I/O could cause a 
stoppage along the entire IO chain and end up causing a delayed ACK 
which might push the connection back into slow start. Even with a 
reasonably fast disk on a fat enough connection seek time can exceed RTT 
and sometimes by several multiples. This just came up at a meeting at 
work and I had no worthwhile answer for them so I'm doing a little 
research. My feeling is that HW disk buffering will prevent it from 
being a factor in 99% of the time. However, I thought I would ask.

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