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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 2.0.0.19 (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.
s->freeze_output=0
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.
Thanks,
Uri.
- [Qemu-devel] migration: adding migration to/from a file (v2), Uri Lublin, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Anthony Liguori, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Uri Lublin, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Anthony Liguori, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2),
Uri Lublin <=
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Anthony Liguori, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Jamie Lokier, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), M. Warner Losh, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Anthony Liguori, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), M. Warner Losh, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Uri Lublin, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Jamie Lokier, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Anthony Liguori, 2009/02/19
- Re: [Qemu-devel] migration: adding migration to/from a file (v2), Jamie Lokier, 2009/02/19