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Re: [Qemu-devel] Safely reopening image files by stashing fds
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] Safely reopening image files by stashing fds |
Date: |
Tue, 09 Aug 2011 11:51:02 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:5.0) Gecko/20110707 Thunderbird/5.0 |
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
- Re: [Qemu-devel] Safely reopening image files by stashing fds, (continued)
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Daniel P. Berrange, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Christoph Hellwig, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Stefan Hajnoczi, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Anthony Liguori, 2011/08/05
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Supriya Kannery, 2011/08/08
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/08
- Re: [Qemu-devel] Safely reopening image files by stashing fds, supriya kannery, 2011/08/09
- Re: [Qemu-devel] Safely reopening image files by stashing fds,
Kevin Wolf <=
- Re: [Qemu-devel] Safely reopening image files by stashing fds, supriya kannery, 2011/08/09
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Supriya Kannery, 2011/08/16
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Supriya Kannery, 2011/08/16
- Re: [Qemu-devel] [RFC] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/17
- Re: [Qemu-devel] Safely reopening image files by stashing fds, Christoph Hellwig, 2011/08/05
Re: [Qemu-devel] Safely reopening image files by stashing fds, Kevin Wolf, 2011/08/05