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Re: [Qemu-devel] Safely reopening image files by stashing fds


From: supriya kannery
Subject: Re: [Qemu-devel] Safely reopening image files by stashing fds
Date: Tue, 09 Aug 2011 15:02:42 +0530
User-agent: Thunderbird 2.0.0.14 (X11/20080501)

Kevin Wolf wrote:
Am 09.08.2011 11:22, schrieb supriya kannery:
Kevin Wolf wrote:
Am 08.08.2011 09:02, schrieb Supriya Kannery:
On 08/05/2011 09:19 PM, Anthony Liguori wrote:
On 08/05/2011 10:43 AM, Kevin Wolf wrote:
Am 05.08.2011 17:24, schrieb Stefan Hajnoczi:
On Fri, Aug 5, 2011 at 3:28 PM, Christoph Hellwig<address@hidden> wrote:
On Fri, Aug 05, 2011 at 02:12:48PM +0100, Daniel P. Berrange wrote:
Because you cannot change O_DIRECT on an open fd :(. This is why
we're going through this pain.
Hmm, I remember hearing that before, but looking at the current
fcntl()
manpage, it claims you *can* change O_DIRECT using SET_FL. Perhaps
this
is a newish feature, but it'd be nicer to use it if possible ?
It's been there since day 1 of O_DIRECT support.
Sorry, my bad. So for Linux we could just use fcntl for
block_set_hostcache and not bother with reopening. However, we will
need to reopen should we wish to support changing O_DSYNC.
We do wish to support that.

Anthony thinks that allowing the guest to toggle WCE is a prerequisite
for making cache=writeback the default. And this is something that I
definitely want to do for 1.0.
Indeed.

We discussed the following so far...
1. How to safely reopen image files
2. Dynamic hostcache change
3. Support for dynamic change of O_DSYNC

Since 2 is independent of 1, shall I go ahead implementing
hostcache change using fcntl.

Implementation for safely reopening image files using "BDRVReopenState"
can be done separately as a pre-requisite before implementing 3
Doing it separately means that we would introduce yet another callback
that is used just to change O_DIRECT. In the end we want it to use
bdrv_reopen(), too, so I'm not sure if there is a need for a temporary
solution.

Could you please explain "In the end we want it to use bdrv_reopen" at bit more. When fcntl() can change O_DIRECT on open fd , is there a need to "re-open"
the image file?

What I meant is that in the end, with a generic bdrv_reopen(), we can
have raw-posix only call dup() and fcntl() instead of doing a
close()/open() sequence if it can satisfy the new flags this way. But
this would be an implementation detail and not be visible in the interface.

Kevin

ok
- thanks, Supriya



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