On Fri, Dec 04, 2020 at 01:27:13AM +0300, Vladimir Sementsov-Ogievskiy wrote: > Finally to be safe with calculations, to not calculate different > maximums for different nodes (depending on cluster size and > request_alignment), let's simply set QEMU_ALIGN_DOWN(INT64_MAX, 2^30) > as absolute maximum bytes length for Qemu. Actually, it's not much less > than INT64_MAX.
> +/* > + * We want allow aligning requests and disk length up to any 32bit alignment > + * and don't afraid of overflow. > + * To achieve it, and in the same time use some pretty number as maximum disk > + * size, let's define maximum "length" (a limit for any offset/bytes request > and > + * for disk size) to be the greatest power of 2 less than INT64_MAX. > + */ > +#define BDRV_MAX_ALIGNMENT (1L << 30) > +#define BDRV_MAX_LENGTH (QEMU_ALIGN_DOWN(INT64_MAX, BDRV_MAX_ALIGNMENT)) This change broke nbdkit tests. We test that qemu can handle a qemu NBD export of size 2^63 - 512, the largest size that (experimentally) we found qemu could safely handle. eg: https://github.com/libguestfs/nbdkit/blob/master/tests/test-memory-largest-for-qemu.sh Before this commit: $ nbdkit memory $(( 2**63 - 512 )) --run './qemu-img info "$uri"' image: nbd://localhost:10809 file format: raw virtual size: 8 EiB (9223372036854775296 bytes) disk size: unavailable After this commit: $ nbdkit memory $(( 2**63 - 512 )) --run './qemu-img info "$uri"' qemu-img: Could not open 'nbd://localhost:10809': Could not refresh total sector count: File too large Can I confirm that this limit is now the new official one and we should adjust nbdkit tests? Or was this change unintentional given that qemu seemed happy to handle 2^63 - 512 disks before? Note that nbdkit & libnbd support up to 2^63 - 1 bytes (we are not limited to whole sectors). Also the Linux kernel will let you create a /dev/nbdX device of size 2^63 - 1. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
