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7843 get_clones_stat() is suboptimal for lots of clones

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illumos/illumos-gate@c5bde7273e
c5bde7273e

https://www.illumos.org/issues/7843
  get_clones_stat() could be very slow if a snapshot has many (thousands) clones.
  Clone names are added to an nvlist that's created with NV_UNIQUE_NAME.
  So, each time a new name is appended to the list, the whole list is searched
  linearly to see if that name is not already in the list. That results in the
  quadratic complexity.
  That should be easy to fix as we know in advance that we should not get any
  duplicate names, so we can drop NV_UNIQUE_NAME when creating the list.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Andriy Gapon <avg@FreeBSD.org>
This commit is contained in:
Andriy Gapon 2017-03-08 13:39:22 +00:00
parent dbac7ab9c0
commit f1a99e4268
1 changed files with 12 additions and 1 deletions
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13
uts/common/fs/zfs/dsl_dataset.c
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@ -1725,10 +1725,21 @@ get_clones_stat(dsl_dataset_t *ds, nvlist_t *nv)
zap_cursor_t zc;
zap_attribute_t za;
nvlist_t *propval = fnvlist_alloc();
nvlist_t *val = fnvlist_alloc();
nvlist_t *val;
ASSERT(dsl_pool_config_held(ds->ds_dir->dd_pool));
/*
* We use nvlist_alloc() instead of fnvlist_alloc() because the
* latter would allocate the list with NV_UNIQUE_NAME flag.
* As a result, every time a clone name is appended to the list
* it would be (linearly) searched for for a duplicate name.
* We already know that all clone names must be unique and we
* want avoid the quadratic complexity of double-checking that
* because we can have a large number of clones.
*/
VERIFY0(nvlist_alloc(&val, 0, KM_SLEEP));
/*
* There may be missing entries in ds_next_clones_obj
* due to a bug in a previous version of the code.