search for: falloc_fl_insert_range

On Fri, Apr 24, 2020 at 02:33:13PM +0200, Michal Privoznik wrote: > On 4/24/20 6:38 AM, Vincent Wu wrote: > > > > The save format is fragile. At the beginning there is a header which > describes the file, then there is libvirt section (which contains the domain > XML and a cookie) and then there is QEMU section (where QEMU saved the guest > memory). Because of this, we

...> > We've had guest XML reaching 1M before, but I agree that the initial > saved image creation should include padding to a nice boundary to make > future edits less likely to overflow the reserved heading. > > On new enough Linux, some file systems support > fallocate(FALLOC_FL_INSERT_RANGE) which can splice in a hole (all later > file contents are shifted in offsets); maybe our save code could take > advantage of that to repair existing saved images with insufficient > header size in a more efficient manner than manually shifting the rest > of the file contents oursel...

...worry about hitting a size limit. We've had guest XML reaching 1M before, but I agree that the initial saved image creation should include padding to a nice boundary to make future edits less likely to overflow the reserved heading. On new enough Linux, some file systems support fallocate(FALLOC_FL_INSERT_RANGE) which can splice in a hole (all later file contents are shifted in offsets); maybe our save code could take advantage of that to repair existing saved images with insufficient header size in a more efficient manner than manually shifting the rest of the file contents ourselves. -- Eric Blake...