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Optimize creation and removal of temporary "user holds" placed on
snapshots by a zfs send, by ensuring all the required holds and
releases are done in a single dsl_sync_task.
Creation now collates the required holds during a dry run and
then uses a single lzc_hold call via zfs_hold_apply instead of
processing each snapshot in turn.
Defered (on exit) cleanup by the kernel is also now done in
dsl_sync_task by reusing dsl_dataset_user_release.
On a test with 11 volumes in a tree each with 8 snapshots on a
single HDD zpool this reduces the time required to perform a full
send from 20 seconds to under 0.8 seconds.
For reference eliminating the hold entirely reduces this 0.15
While I'm here:-
* Remove some unused structures
* Fix nvlist_t leak in zfs_release_one

*** 4978,4997 **** */ /* ARGSUSED */ static int zfs_ioc_release(const char *pool, nvlist_t *holds, nvlist_t *errlist) { - nvpair_t *pair; - - /* - * The release may cause the snapshot to be destroyed; make sure it - * is not mounted. - */ - for (pair = nvlist_next_nvpair(holds, NULL); pair != NULL; - pair = nvlist_next_nvpair(holds, pair)) - zfs_unmount_snap(nvpair_name(pair)); - return (dsl_dataset_user_release(holds, errlist)); } /* * inputs: --- 4978,4987 ----