On Wed, Jul 08, 2020 at 07:17:52PM +0300, Vladimir Sementsov-Ogievskiy wrote: > 08.07.2020 15:07, Stefan Hajnoczi wrote: > > On Sat, Jun 20, 2020 at 05:36:47PM +0300, Vladimir Sementsov-Ogievskiy > > wrote: > > > It may be used for file-systems with slow allocation. > > > > > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> > > > --- > > > qapi/block-core.json | 3 +- > > > block/preallocate.c | 255 +++++++++++++++++++++++++++++++++++++++++++ > > > block/Makefile.objs | 1 + > > > 3 files changed, 258 insertions(+), 1 deletion(-) > > > create mode 100644 block/preallocate.c > > > > Please add documentation to docs/system/qemu-block-drivers.rst.inc > > describing the purpose of this block driver and how to use it. > > This implies adding new section "Filters", yes?
Yes, please. > > > > Since this filter grows the file I guess it's intended to be below an > > image format? > > Yes, between format and protocol nodes. Thanks for confirming. Please include this in the documentation. While reading the code I was thinking about how this can work if the guest is directly exposed to the filter. Users might wonder about the same thing. > > > + /* Length should not have changed */ > > > + assert(len == bdrv_getlength(bs->file->bs)); > > > + > > > + start = write_zero ? MIN(offset, len) : len; > > > + end = QEMU_ALIGN_UP(offset + bytes + s->prealloc_size, > > > s->prealloc_align); > > > + > > > + ret = bdrv_co_pwrite_zeroes_locked(bs->file, start, end - start, > > > + BDRV_REQ_NO_FALLBACK, lock); > > > + > > > + bdrv_co_range_unlock(lock); > > > > Hmm...if this piece of code is the only user of bdrv_co_range_try_lock() > > then a BDRV_REQ_NO_WAIT flag might be a simpler API. > > > > I thought the lock request would be used to perform multiple operations, > > but if it's just for a single operation then I think it's less code and > > easier to understand without the lock request. > > Hmm, again, I don't remember exact reasons. Firstly, I was afraid of length > change during try_lock and have a double check for bdrv_getlength(). Then > I decided that it's impossible and change the check to an assertion. > Probably, the only reason to leave locked range was "I already have the code, > it will help with copy-on-read, why not to use it".. OK, I'll try rewrite it > with help of new flag. Thanks! Stefan
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