Re: [PATCH 1/1] prealloc: add truncate mode for prealloc filter

[Denis V. Lunev]

On 5/29/24 04:33, Wang, Lei wrote:
> On 5/1/2024 1:05, Denis V. Lunev via wrote:
> > Preallocate filter allows to implement really interesting setups.
> >
> > Assume that we have
> > * shared block device, f.e. iSCSI LUN, implemented with some HW device
> > * clustered LVM on top of it
> > * QCOW2 image stored inside LVM volume
> >
> > This allows very cheap clustered setups with all QCOW2 features intact.
> > Currently supported setups using QCOW2 with data_file option are not
> > so cool as snapshots are not allowed, QCOW2 should be placed into some
> > additional distributed storage and so on.
> >
> > Though QCOW2 inside LVM volume has a drawback. The image is growing and
> > in order to accomodate that image LVM volume is to be resized. This
> > could be done externally using ENOSPACE event/condition but this is
> > cumbersome.
> >
> > This patch introduces native implementation for such a setup. We should
> > just put prealloc filter in between QCOW2 format and file nodes. In that
> > case LVM will be resized at proper moment and that is done effectively
> > as resizing is done in chinks.
> >
> > The patch adds allocation mode for this purpose in order to distinguish
> > 'fallocate' for ordinary file system and 'truncate'.
> >
> > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > CC: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
> > CC: Kevin Wolf <kwolf@redhat.com>
> > CC: Hanna Reitz <hreitz@redhat.com>
> > CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> > ---
> >   block/preallocate.c | 50 +++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 48 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/preallocate.c b/block/preallocate.c
> > index 4d82125036..6d31627325 100644
> > --- a/block/preallocate.c
> > +++ b/block/preallocate.c
> > @@ -33,10 +33,24 @@
> >   #include "block/block-io.h"
> >   #include "block/block_int.h"
> >   
> > +typedef enum PreallocateMode {
> > +    PREALLOCATE_MODE_FALLOCATE = 0,
> > +    PREALLOCATE_MODE_TRUNCATE = 1,
> > +    PREALLOCATE_MODE__MAX = 2,
> > +} PreallocateMode;
> > +
> > +static QEnumLookup prealloc_mode_lookup = {
> > +    .array = (const char *const[]) {
> > +        "falloc",
> > +        "truncate",
> > +    },
> > +    .size = PREALLOCATE_MODE__MAX,
> > +};
> >   
> >   typedef struct PreallocateOpts {
> >       int64_t prealloc_size;
> >       int64_t prealloc_align;
> > +    PreallocateMode prealloc_mode;
> >   } PreallocateOpts;
> >   
> >   typedef struct BDRVPreallocateState {
> > @@ -79,6 +93,7 @@ typedef struct BDRVPreallocateState {
> >   
> >   #define PREALLOCATE_OPT_PREALLOC_ALIGN "prealloc-align"
> >   #define PREALLOCATE_OPT_PREALLOC_SIZE "prealloc-size"
> > +#define PREALLOCATE_OPT_MODE "mode"
> Why not keeping the names consistent, I mean:
>
> #define PREALLOCATE_OPT_PREALLOC_MODE "prealloc-mode"
I have had some thoughts on that, like prealloc-mode
name is already used on the generic level, but obviously
I have mistaken.

Sure thing, I'll rename to "prealloc-mode" and in the
same way in the macro on next submission.

Thanks!