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

[Stefano Garzarella]

On Wed, May 08, 2019 at 01:44:27PM +0200, Markus Armbruster wrote:
> Stefano Garzarella <address@hidden> writes:
> 
> > 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!
> 
> 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.

If you agree, I'll put it as a background task, until somebody from
management applications tell us his interest.

> 
> >> 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.
> 
> Yes, please!

Okay, I'll send a patch to update it.

Thanks,
Stefano