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There are some optimizations that require knowing if an image starts out as reading all zeroes, such as making blockdev-mirror faster by skipping the copying of source zeroes to the destination. The existing bdrv_co_is_zero_fast() is a good building block for answering this question, but it tends to give an answer of 0 for a file we just created via QMP 'blockdev-create' or similar (such as 'qemu-img create -f raw'). Why? Because file-posix.c insists on allocating a tiny header to any file rather than leaving it 100% sparse, due to some filesystems that are unable to answer alignment probes on a hole. But teaching file-posix.c to read the tiny header doesn't scale - the problem of a small header is also visible when libvirt sets up an NBD client to a just-created file on a migration destination host. So, we need a wrapper function that handles a bit more complexity in a common manner for all block devices - when the BDS is mostly a hole, but has a small non-hole header, it is still worth the time to read that header and check if it reads as all zeroes before giving up and returning a pessimistic answer. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-ID: <20250509204341.3553601-19-eblake@redhat.com> |
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| .. | ||
| accounting.h | ||
| aio-wait.h | ||
| aio.h | ||
| aio_task.h | ||
| block-common.h | ||
| block-copy.h | ||
| block-global-state.h | ||
| block-hmp-cmds.h | ||
| block-io.h | ||
| block.h | ||
| block_backup.h | ||
| block_int-common.h | ||
| block_int-global-state.h | ||
| block_int-io.h | ||
| block_int.h | ||
| blockjob.h | ||
| blockjob_int.h | ||
| dirty-bitmap.h | ||
| export.h | ||
| fuse.h | ||
| graph-lock.h | ||
| nbd.h | ||
| nvme.h | ||
| qapi.h | ||
| qdict.h | ||
| raw-aio.h | ||
| replication.h | ||
| reqlist.h | ||
| snapshot.h | ||
| thread-pool.h | ||
| throttle-groups.h | ||
| ufs.h | ||
| write-threshold.h | ||