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Re: [Qemu-block] [Qemu-devel] [PATCH] block/gluster: add support for SEE

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE
Date: Thu, 6 Oct 2016 17:09:59 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 03/07/2016 01:14 PM, Eric Blake wrote:
> [adding qemu-devel; ALL patches must cc qemu-devel even when sent to
> another list]
> On 03/07/2016 11:04 AM, 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.
>> ---

I hit a weird failure when trying to compile this on an older RHEL 6
box, where /usr/include/unistd.h is too old to include SEEK_DATA and

block/gluster.c: In function ‘qemu_gluster_test_seek’:
block/gluster.c:684: error: ‘SEEK_DATA’ undeclared (first use in this
block/gluster.c:684: error: (Each undeclared identifier is reported only
block/gluster.c:684: error: for each function it appears in.)
block/gluster.c: In function ‘find_allocation’:
block/gluster.c:1202: error: ‘SEEK_DATA’ undeclared (first use in this
block/gluster.c:1234: error: ‘SEEK_HOLE’ undeclared (first use in this

The patch has been in place for several months (which shows how seldom I
compile on that particular box), but it makes me wonder why none of the
autobuilders have hit this failure.  But since the code mentions that it
shamelessly copies from raw-posix.c, and that file in turn has #ifdef
guards to only do SEEK_HOLE optimizations if the system headers defined
SEEK_HOLE in the first place, it sounds like you need to do a followup
patch along those lines.

>> + *
>> + * (Shamefully copied from raw-posix.c, only miniscule adaptions.)
>> + */
>> +static int find_allocation(BlockDriverState *bs, off_t start,
>> +                           off_t *data, off_t *hole)
>> +{
>> +    BDRVGlusterState *s = bs->opaque;
>> +    off_t offs;
>> +
>> +    /*
>> +     * SEEK_DATA cases:
>> +     * D1. offs == start: start is in data
>> +     * D2. offs > start: start is in a hole, next data at offs
>> +     * D3. offs < 0, errno = ENXIO: either start is in a trailing hole
>> +     *                              or start is beyond EOF
>> +     *     If the latter happens, the file has been truncated behind
>> +     *     our back since we opened it.  All bets are off then.
>> +     *     Treating like a trailing hole is simplest.
>> +     * D4. offs < 0, errno != ENXIO: we learned nothing
>> +     */
>> +    offs = glfs_lseek(s->fd, start, SEEK_DATA);

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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