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Re: [Qemu-devel] migration: adding migration to/from a file (v2)

From: Jamie Lokier
Subject: Re: [Qemu-devel] migration: adding migration to/from a file (v2)
Date: Thu, 19 Feb 2009 19:37:33 +0000
User-agent: Mutt/1.5.13 (2006-08-11)

Anthony Liguori wrote:
> Uri Lublin wrote:
> >Anthony Liguori wrote:
> >>Uri Lublin wrote:
> >>>
> >>>Migration to file, uses migration-to-fd (supports live migration).
> >>>Migration from file, uses qemu-fopen directly.
> >>
> >>Eh?  Haven't we already talked about why this doesn't work?  Maybe 
> >>there's a v3 that you meant to send?
> >>
> >
> >Actually I do have a v3 which uses posix-aio-compat.c
> >It's a much more complicated solution then just writing to a file though.
> >Also I am not sure if I need to use a signal or not as the migration 
> >(to-fd) code is polling. And if I use signal should I use SIGUSR2 or a 
> >different one and use a pipe similar to block-raw-posix.c ?
> How is the migration code polling?  It will attempt to do writes until a 
> write returns EAGAIN.  At this point, it will wait for notification that 
> the more writes are available. 

> Remember, migration is a streaming 
> protocol, not a random access, so it only makes sense to have one 
> outstanding request at a time.

Fwow, often with streaming (in general) the highest performance comes
from having two or more outstanding requests at a time, so there are
no gaps between requests being processed at the OS and device level.

However, unless migrate-to-file uses O_DIRECT that will be hidden by
the OS's buffering.

Then the remaining overhead from AIOs (or fake AIOs using threads) is
lots context switching, one per write completed.

-- Jamie

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