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

From: Uri Lublin
Subject: Re: [Qemu-devel] migration: adding migration to/from a file (v2)
Date: Thu, 19 Feb 2009 21:06:35 +0200
User-agent: Thunderbird (X11/20090105)

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.

Your code would look something like:

write() -> submit aio request
until aio completes, write returns EAGAIN
when aio completes, notify migration code that we are writable again

Basically that's what I've done in my v3.
But since I fake EAGAIN, and the fd is writeable by select, the scenario is as follows:

file_write (s->write of fd-migration) returns EAGAIN
migrate_fd_put_buffer calls qemu_set_fd_handler2 with migrate_fd_put_notify
upon the next main_loop_wait fd is found writable and migrate_fd_put_notify is called.
qemu_file_put_notify is called.
buffered_put_buffer is called.
buffered_flush is called.
migrate_fd_put_buffer is called
file_write is called again -- here I can check if previous aio write finished.

Your concern in the previous patch was that write() to a regular file might block. That's why I'm calling select before calling write. Do you think select will mark the fd as writeable but write would still block ?

select() doesn't help. It will return that the file descriptor is writable and then the subsequent write will block.

Sure looks like a bug.
I'll send my v3 using posix-aio-compat for review soon.


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