Re: [Qemu-devel] Use of PreallocMode in block drivers

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, May 09, 2019 at 02:07:34PM +0200, Markus Armbruster wrote:
> Stefano Garzarella <sgarzare@redhat.com> writes:
> 
> > On Wed, May 08, 2019 at 01:44:27PM +0200, Markus Armbruster wrote:
> >> Stefano Garzarella <sgarzare@redhat.com> writes:
> >> 
> >> > On Tue, May 07, 2019 at 08:34:51AM +0200, Markus Armbruster wrote:
> >> >> Cc: Peter for a libvirt perspective.
> >> >> 
> >> >> Stefano Garzarella <sgarzare@redhat.com> writes:
> >> >> 
> >> >> > This patch adds the support of preallocation (off/full) for the RBD
> >> >> > block driver.
> >> >> > If available, we use rbd_writesame() to quickly fill the image when
> >> >> > full preallocation is required.
> >> >> >
> >> >> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> >> >> > ---
> >> >> >  block/rbd.c          | 149 ++++++++++++++++++++++++++++++++++++++-----
> >> >> >  qapi/block-core.json |   4 +-
> >> >> >  2 files changed, 136 insertions(+), 17 deletions(-)
> >> >> >
> >> >> > diff --git a/block/rbd.c b/block/rbd.c
> >> >> > index 0c549c9935..29dd1bb040 100644
> >> >> > --- a/block/rbd.c
> >> >> > +++ b/block/rbd.c
> >> >> > @@ -13,6 +13,7 @@
> >> >> >  
> >> >> >  #include "qemu/osdep.h"
> >> >> >  
> >> >> > +#include "qemu/units.h"
> >> >> >  #include <rbd/librbd.h>
> >> >> >  #include "qapi/error.h"
> >> >> >  #include "qemu/error-report.h"
> >> >> > @@ -331,6 +332,110 @@ static void qemu_rbd_memset(RADOSCB *rcb, int64_t offs)
> >> >> >      }
> >> >> >  }
> >> >> >  
> >> >> > +static int qemu_rbd_do_truncate(rbd_image_t image, int64_t offset,
> >> >> > +                                PreallocMode prealloc, Error **errp)
> >> >> > +{
> >> >> > +    uint64_t current_length;
> >> >> > +    char *buf = NULL;
> >> >> > +    int ret;
> >> >> > +
> >> >> > +    ret = rbd_get_size(image, &current_length);
> >> >> > +    if (ret < 0) {
> >> >> > +        error_setg_errno(errp, -ret, "Failed to get file length");
> >> >> > +        goto out;
> >> >> > +    }
> >> >> > +
> >> >> > +    if (current_length > offset && prealloc != PREALLOC_MODE_OFF) {
> >> >> > +        error_setg(errp, "Cannot use preallocation for shrinking files");
> >> >> > +        ret = -ENOTSUP;
> >> >> > +        goto out;
> >> >> > +    }
> >> >> > +
> >> >> > +    switch (prealloc) {
> >> >> > +    case PREALLOC_MODE_FULL: {
> >> >> [...]
> >> >> > +    case PREALLOC_MODE_OFF:
> >> >> [...]
> >> >> > +    default:
> >> >> > +        error_setg(errp, "Unsupported preallocation mode: %s",
> >> >> > +                   PreallocMode_str(prealloc));
> >> >> > +        ret = -ENOTSUP;
> >> >> > +        goto out;
> >> >> > +    }
> >> >> 
> >> >> Other block drivers also accept only some values of PreallocMode.  Okay.
> >> >> 
> >> >> I wonder whether management applications need to know which values are
> >> >> supported.
> >> >
> >> > Good point!
> >> 
> >> We can continue to assume they don't until somebody tells us otherwise.
> >> 
> >> >> Let me review support in drivers:
> >> >> 
> >> >> * file (file-win32.c)
> >> >> * iscsi
> >> >> * nfs
> >> >> * qed
> >> >> * ssh
> >> >> 
> >> >>   - Reject all but PREALLOC_MODE_OFF
> >> >> 
> >> >> * copy-on-read
> >> >> * luks (crypto.c)
> >> >> * raw
> >> >> 
> >> >>   - Pass through only
> >> >> 
> >> >> * file host_cdrom host_device (file-posix.c)
> >> >> 
> >> >>   - Reject all but PREALLOC_MODE_OFF when shrinking and for non-regular
> >> >>     files
> >> >>   - Reject PREALLOC_MODE_FALLOC unless CONFIG_POSIX_FALLOCATE
> >> >>   - Reject PREALLOC_MODE_METADATA
> >> >> 
> >> >> * gluster
> >> >> 
> >> >>   - Reject all but PREALLOC_MODE_OFF when shrinking
> >> >>   - Reject PREALLOC_MODE_FALLOC unless CONFIG_GLUSTERFS_FALLOCATE
> >> >>   - Reject PREALLOC_MODE_FULL unless CONFIG_GLUSTERFS_ZEROFILL
> >> >>   - Reject PREALLOC_MODE_METADATA
> >> >> 
> >> >> * qcow2
> >> >> 
> >> >>   - Reject all but PREALLOC_MODE_OFF when shrinking and with a backing
> >> >>     file
> >> >>   
> >> >> * rbd with this patch
> >> >> 
> >> >>   - Reject all but PREALLOC_MODE_OFF when shrinking
> >> >>   - Reject PREALLOC_MODE_METADATA and PREALLOC_MODE_FALLOC
> >> >> 
> >> >> * sheepdog
> >> >> 
> >> >>   - Reject PREALLOC_MODE_METADATA and PREALLOC_MODE_FALLOC
> >> >>   - Doesn't support shrinking
> >> >> 
> >> >> * vdi
> >> >> 
> >> >>   - Reject PREALLOC_MODE_FALLOC and PREALLOC_MODE_FULL
> >> >>   - Doesn't support shrinking
> >> >> 
> >> >> * blkdebug
> >> >> * blklogwrites
> >> >> * blkverify
> >> >> * bochs
> >> >> * cloop
> >> >> * dmg
> >> >> * ftp
> >> >> * ftps
> >> >> * http
> >> >> * https
> >> >> * luks
> >> >> * nbd
> >> >> * null-aio
> >> >> * null-co
> >> >> * nvme
> >> >> * parallels
> >> >> * qcow
> >> >> * quorum
> >> >> * replication
> >> >> * throttle
> >> >> * vhdx
> >> >> * vmdk
> >> >> * vpc
> >> >> * vvfat
> >> >> * vxhs
> >> >> 
> >> >>   - These appear not to use PreallocMode: they don't implement
> >> >>     .bdrv_co_truncate(), and either don't implement .bdrv_co_create() or
> >> >>     implement it without a prealloc parameter.
> >> >> 
> >> >> Looks good to me.
> >> >>
> >> >
> >> > Thanks for the analysis!
> >> >
> >> >> > +
> >> >> > +    ret = 0;
> >> >> > +
> >> >> > +out:
> >> >> > +    g_free(buf);
> >> >> > +    return ret;
> >> >> > +}
> >> >> > +
> >> >> >  static QemuOptsList runtime_opts = {
> >> >> >      .name = "rbd",
> >> >> >      .head = QTAILQ_HEAD_INITIALIZER(runtime_opts.head),
> >> >> [...]
> >> >> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> >> >> > index 7ccbfff9d0..db25a4065b 100644
> >> >> > --- a/qapi/block-core.json
> >> >> > +++ b/qapi/block-core.json
> >> >> > @@ -4277,13 +4277,15 @@
> >> >> >  #                   point to a snapshot.
> >> >> >  # @size             Size of the virtual disk in bytes
> >> >> >  # @cluster-size     RBD object size
> >> >> > +# @preallocation    Preallocation mode (allowed values: off, full)
> >> >> >  #
> >> >> >  # Since: 2.12
> >> >> >  ##
> >> >> >  { 'struct': 'BlockdevCreateOptionsRbd',
> >> >> >    'data': { 'location':         'BlockdevOptionsRbd',
> >> >> >              'size':             'size',
> >> >> > -            '*cluster-size' :   'size' } }
> >> >> > +            '*cluster-size' :   'size',
> >> >> > +            '*preallocation':   'PreallocMode' } }
> >> >> >  
> >> >> >  ##
> >> >> >  # @BlockdevVmdkSubformat:
> >> >> 
> >> >> The non-support of values 'metadata' and 'falloc' is not visible in
> >> >> introspection, only in documentation.  No reason to block this patch, as
> >> >> the other block drivers have the same introspection weakness (only
> >> >> sheepdog and vdi bother to document).
> >> >> 
> >> >> Should we address the introspection weakness?  Only if there's a use for
> >> >> the information, I think.
> >> >
> >> > If the management applications will use that information (or maybe also
> >> > our help pages), could be useful to have an array of 'PreallocMode'
> >> > supported per-driver.
> >> 
> >> Ideally, query-qmp-schema would show only the supported values.
> >> 
> >> Not hard to do, just tedious: we'd get a number of sub-enums in addition
> >> to the full one, and we'd have to map from sub-enum to the full one.
> >> 
> >> QAPI language support for sub-enums would remove most of the tedium.
> >> Not worthwhile unless the need for sub-enums is actually common.
> >
> > I should study better the QMP and QAPI to understand how to implement
> > the sub-enums.
> 
> Sub-enums of
> 
>     { 'enum': 'PreallocMode',
>       'data': [ 'off', 'metadata', 'falloc', 'full' ] }
> 
> done the obvious way:
> 
>     { 'enum': 'PreallocModeOff',
>       'data': [ 'off' ] }
>     { 'enum': 'PreallocModeOffPosix',
>       'data': [ 'off', 'metadata',
>                  { 'name': 'falloc', 'if': 'defined(CONFIG_POSIX_FALLOCATE)' },
>                  'full' ] }
> 
> and so forth.
> 
> This generates a bunch of different C enum types in addition to
> PreallocMode: PreallocModeOff, PreallocModePosix, ...
> 
> Common C code continues to use just PreallocMode.  The QMP command
> handlers using sub-enums will have to map between the sub-enums and
> PreallocMode.
> 
> Tedious.
> 
> With QAPI language support for sub-enums, we could eliminate the
> additional C enums.
> 

Okay, I understood your idea.
Thanks for the explanation!

> > If you agree, I'll put it as a background task, until somebody from
> > management applications tell us his interest.
> 
> Only act if there's a compelling use case.

Sure.

Thanks,
Stefano