Bug #7430

Backfill metadnode more intelligently

Added by Alek Pinchuk over 4 years ago. Updated about 4 years ago.

Start date:
Due date:
% Done:


Estimated time:
Gerrit CR:


Description and patch from brought over from the following ZoL commit:

Only attempt to backfill lower metadnode object numbers if at least
4096 objects have been freed since the last rescan, and at most once
per transaction group. This avoids a pathology in dmu_object_alloc()
that caused O(N^2) behavior for create-heavy workloads and
substantially improves object creation rates. As summarized by
Matthew Ahrens in #4636:

"Normally, the object allocator simply checks to see if the next
object is available. The slow calls happened when dmu_object_alloc()
checks to see if it can backfill lower object numbers. This happens
every time we move on to a new L1 indirect block (i.e. every 32 *
128 = 4096 objects). When re-checking lower object numbers, we use
the on-disk fill count (blkptr_t:blk_fill) to quickly skip over
indirect blocks that don’t have enough free dnodes (defined as an L2
with at least 393,216 of 524,288 dnodes free). Therefore, we may
find that a block of dnodes has a low (or zero) fill count, and yet
we can’t allocate any of its dnodes, because they've been allocated
in memory but not yet written to disk. In this case we have to hold
each of the dnodes and then notice that it has been allocated in

The end result is that allocating N objects in the same TXG can
require CPU usage proportional to N^2."

Add a tunable dmu_rescan_dnode_threshold to define the number of
objects that must be freed before a rescan is performed. Don't bother
to export this as a module option because testing doesn't show a
compelling reason to change it. The vast majority of the performance
gain comes from limit the rescan to at most once per TXG.


Updated by Electric Monk about 4 years ago

  • Status changed from New to Closed
  • % Done changed from 80 to 100

git commit af346df58864e8fe897b1ff1a3a4c12f9294391b

commit  af346df58864e8fe897b1ff1a3a4c12f9294391b
Author: Ned Bass <>
Date:   2016-10-22T03:22:32.000Z

    7430 Backfill metadnode more intelligently
    Reviewed by: Alek Pinchuk <>
    Reviewed by: Brian Behlendorf <>
    Reviewed by: Matthew Ahrens <>
    Approved by: Gordon Ross <>

Also available in: Atom PDF