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Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEE
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE |
Date: |
Tue, 8 Mar 2016 13:53:26 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 08.03.2016 um 05:21 hat Niels de Vos geschrieben:
> On Mon, Mar 07, 2016 at 01:27:38PM -0500, Jeff Cody wrote:
> > On Mon, Mar 07, 2016 at 07:04:15PM +0100, Niels de Vos wrote:
> > > GlusterFS 3.8 contains support for SEEK_DATA and SEEK_HOLE. This makes
> > > it possible to detect sparse areas in files.
> > >
> > > Signed-off-by: Niels de Vos <address@hidden>
> > >
> > > --
> > > Tested by compiling and running "qemu-img map gluster://..." with a
> > > build of the current master branch of glusterfs. Using a Fedora
> > > cloud image (in raw format) shows many SEEK procudure calls going back
> > > and forth over the network. The output of "qemu map" matches the output
> > > when run against the image on the local filesystem.
> > > ---
> > > block/gluster.c | 159
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > configure | 25 +++++++++
> > > 2 files changed, 184 insertions(+)
> > >
> > > diff --git a/block/gluster.c b/block/gluster.c
> > > index 65077a0..1430010 100644
> > > --- a/block/gluster.c
> > > +++ b/block/gluster.c
> > > @@ -677,6 +677,153 @@ static int
> > > qemu_gluster_has_zero_init(BlockDriverState *bs)
> > > return 0;
> > > }
> > >
> > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> >
> > Why do we need to make this a compile-time option? Version checking
> > is problematic; for instance, different distributions may have
> > backported bug fixes / features, that are not reflected by the
> > reported version number, etc.. Ideally, we can determine
> > functionality during runtime, and behave accordingly.
>
> This will not get backported to older Gluster versions, it required a
> protocol change.
>
> > If SEEK_DATA and SEEK_HOLE are not supported,
> > qemu_gluster_co_get_block_status can return that sectors are all
> > allocated (which is what happens in block/io.c anyway if the driver
> > doesn't support the function).
>
> Ok, good to know.
>
> > As long as glfs_lseek() will return error (e.g. EINVAL) for an invalid
> > whence value, we can handle it runtime. Does glfs_lseek() behave
> > sanely?
>
> Unfortunately older versions of libgfapi do not return EINVAL when
> SEEK_DATA/HOLE is used. It is something we'll need to fix in the stable
> releases. We can not assume that all users have installed a version of
> the library that handles SEEK_DATA/HOLE correctly (return EINVAL) when
> there is no support in the network protocol or on the server.
>
> To be sure that we don't get some undefined behaviour, the compile time
> check is needed.
The code could be compiled on a host with newer libgfapi, but run on a
different host with an older version. This is why having (only) compile
time checks is rarely a good idea.
Jeff's suggestion to probe the actual behaviour on the host we're
running on in .bdrv_open() sounds reasonable to me.
Kevin
- Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE, (continued)
- Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Jeff Cody, 2016/03/08
- Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Niels de Vos, 2016/03/08
- [Qemu-devel] [PATCH v2] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Niels de Vos, 2016/03/09
- Re: [Qemu-devel] [PATCH v2] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Jeff Cody, 2016/03/09
- Re: [Qemu-devel] [PATCH v2] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Niels de Vos, 2016/03/09
- Re: [Qemu-devel] [PATCH v2] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Jeff Cody, 2016/03/09
- [Qemu-devel] [PATCH v3] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Niels de Vos, 2016/03/10
- Re: [Qemu-devel] [PATCH v3] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Jeff Cody, 2016/03/15
- Re: [Qemu-devel] [PATCH v3] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Jeff Cody, 2016/03/15
- Re: [Qemu-devel] [PATCH v3] block/gluster: add support for SEEK_DATA/SEEK_HOLE, Niels de Vos, 2016/03/16
Re: [Qemu-devel] [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE,
Kevin Wolf <=