22.01.2019 21:57, Kevin Wolf wrote: > Am 11.01.2019 um 12:40 hat Vladimir Sementsov-Ogievskiy geschrieben: >> 11.01.2019 13:41, Kevin Wolf wrote: >>> Am 10.01.2019 um 14:20 hat Vladimir Sementsov-Ogievskiy geschrieben: >>>> drv_co_block_status digs bs->file for additional, more accurate search >>>> for hole inside region, reported as DATA by bs since 5daa74a6ebc. >>>> >>>> This accuracy is not free: assume we have qcow2 disk. Actually, qcow2 >>>> knows, where are holes and where is data. But every block_status >>>> request calls lseek additionally. Assume a big disk, full of >>>> data, in any iterative copying block job (or img convert) we'll call >>>> lseek(HOLE) on every iteration, and each of these lseeks will have to >>>> iterate through all metadata up to the end of file. It's obviously >>>> ineffective behavior. And for many scenarios we don't need this lseek >>>> at all. >>>> >>>> So, let's "5daa74a6ebc" by default, leaving an option to return >>>> previous behavior, which is needed for scenarios with preallocated >>>> images. >>>> >>>> Add iotest illustrating new option semantics. >>>> >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]> >>> >>> I still think that an option isn't a good solution and we should try use >>> some heuristics instead. >> >> Do you think that heuristics would be better than fair cache for lseek >> results? > > I just played a bit with this (qemu-img convert only), and how much > caching lseek() results helps depends completely on the image. As it > happened, my test image was the worst case where caching didn't buy us > much. Obviously, I can just as easily construct an image where it makes > a huge difference. I think that most real-world images should be able to > take good advantage of it, though, and it doesn't hurt, so maybe that's > a first thing that we can do in any case. It might not be the complete > solution, though.
Hmm, and one more idea from Den: We can detect preallocated image, comparing allocated size of real file with number of non-zero qcow2 refcounts. So, real allocation is much less than allocation in qcow2 point of view, we'll enable lseeks, otherwise - not. -- Best regards, Vladimir
