Re: [Nbd] question about i/o scheduling
- To: Peter Simons <simons@...813...>
- Cc: email@example.com
- Subject: Re: [Nbd] question about i/o scheduling
- From: Wouter Verhelst <w@...112...>
- Date: Tue, 3 Aug 2010 23:22:51 -0400
- Message-id: <20100804032250.GF3885@...510...>
- In-reply-to: <87k4o71sk2.fsf@...814...>
- References: <87k4o71sk2.fsf@...814...>
On Tue, Aug 03, 2010 at 08:04:29PM +0200, Peter Simons wrote:
> Hi guys,
> first of all, thanks a lot for the effort you're putting into nbd. The
> package has handled my off-site backups gracefully for 2+ years, so I am
> one happy user!
> Now, it looks to me like my kernel performs too much write-buffering.
> When a large file is copied to an nbd-mounted partition, the initial 2
> MB or so are transferred at incredible speed; apparently, because the
> kernel just buffers the data. At one point, the kernel actually tries to
> transfer the data to the remote host, but that takes several minutes.
> During that time, the nbd-device is almost unusable for other processes
> because it accepts no more writes at all (and reads have a hard time
> getting through the network connection).
> Has anyone else observed that phenomenon? Is there maybe a way to
> improve the situation?
You can try to change the I/O scheduler. The default for most devices is
"cfq", but that isn't very useful for nbd. The 'deadline' one could
yield better results.
The biometric identification system at the gates of the CIA headquarters
works because there's a guard with a large gun making sure no one is
trying to fool the system.