Re: [Qemu-block] [Qemu-devel] Use of PreallocMode in block drivers (was: [PATCH] block/rbd: add preallocation support)

[Stefano Garzarella]

On Tue, May 07, 2019 at 08:34:51AM +0200, Markus Armbruster wrote:
> Cc: Peter for a libvirt perspective.
> 
> Stefano Garzarella <address@hidden> 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 <address@hidden>
> > ---
> >  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!

> 
> 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.

> 
> Should we improve documentation for the other block drivers?
> 

Yes, e.g. for Gluster it is not updated.
If you agree, I can check and update the documentation of all drivers following
your analysis.

Cheers,
Stefano