From: "Richard W.M. Jones" <rjones@redhat.com>
To: richard -rw- weinberger <richard.weinberger@gmail.com>
Cc: "user-mode-linux-devel@lists.sourceforge.net"
<user-mode-linux-devel@lists.sourceforge.net>
Subject: Re: [uml-devel] Performance tests (was: Using UML in libguestfs)
Date: Thu, 15 Aug 2013 19:59:10 +0100 [thread overview]
Message-ID: <20130815185910.GQ5918@redhat.com> (raw)
In-Reply-To: <CAFLxGvzaJOtW-evuHvh9Ue+wcYO7BnoGg4dNWp7mYygo4t6gUg@mail.gmail.com>
On Thu, Aug 15, 2013 at 08:41:53PM +0200, richard -rw- weinberger wrote:
> Meant regarding fsync()...
Ah right, see what you mean :-)
It calls sync(2). Then it opens each /dev/ubdX device and calls fsync
on the file descriptor:
https://github.com/libguestfs/libguestfs/blob/master/daemon/sync.c#L54
The reason for this is a bit complicated, but has to do with write
barriers. I believe this has been fixed since then.
https://github.com/libguestfs/libguestfs/commit/c0a3c9ce70b98171e737e49e6dccc4457963f2ec
In any case, we're calling sync & fsync in the guest, and that ain't
causing the host cache to be flushed.
It's pretty easy to show this with libguestfs:
LIBGUESTFS_BACKEND=uml LIBGUESTFS_HV=~/d/linux-um/linux \
time ./run ./fish/guestfish -N fs:ext2:1G -m /dev/sda1 fallocate64 /data 800M
time sync
The first command finishes in 8 seconds, with barely any disk
activity. The sync afterwards takes 9 seconds with the disk light on
the whole time, while it actually writes the whole 800 MB of data.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-top is 'top' for virtual machines. Tiny program with many
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next prev parent reply other threads:[~2013-08-15 18:59 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-09 20:43 [uml-devel] Using UML in libguestfs Richard W.M. Jones
2013-08-09 20:57 ` richard -rw- weinberger
2013-08-09 21:31 ` Richard W.M. Jones
2013-08-10 4:16 ` Mattia Dongili
2013-08-09 21:15 ` Richard W.M. Jones
2013-08-09 21:18 ` Richard Weinberger
2013-08-09 21:24 ` Richard W.M. Jones
2013-08-11 19:05 ` Geert Uytterhoeven
2013-08-11 19:27 ` Richard W.M. Jones
2013-08-13 11:16 ` Geert Uytterhoeven
2013-08-14 17:59 ` [uml-devel] Performance tests (was: Using UML in libguestfs) Richard W.M. Jones
2013-08-15 17:37 ` richard -rw- weinberger
2013-08-15 17:59 ` Richard W.M. Jones
2013-08-15 18:41 ` richard -rw- weinberger
2013-08-15 18:59 ` Richard W.M. Jones [this message]
2013-08-15 20:31 ` richard -rw- weinberger
2013-08-16 7:54 ` Richard W.M. Jones
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