Hello all,

  I understand that relatively high fragmentation is inherent
to ZFS due to its COW and possible intermixing of metadata
and data blocks (of which metadata path blocks are likely
to expire and get freed relatively quickly).

  I believe it was sometimes implied on this list that such
fragmentation for "static" data can be currently combatted
only by zfs send-ing existing pools data to other pools at
some reserved hardware, and then clearing the original pools
and sending the data back. This is time-consuming, disruptive
and requires lots of extra storage idling for this task (or
at best - for backup purposes).

  I wonder how resilvering works, namely - does it write
blocks "as they were" or in an optimized (defragmented)
fashion, in two usecases:
1) Resilvering from a healthy array (vdev) onto a spare drive
   in order to replace one of the healthy drives in the vdev;
2) Resilvering a degraded array from existing drives onto a
   new drive in order to repair the array and make it redundant

Also, are these two modes different at all?
I.e. if I were to ask ZFS to replace a working drive with
a spare in the case (1), can I do it at all, and would its
data simply be copied over, or reconstructed from other
drives, or some mix of these two operations?

  Finally, what would the gurus say - does fragmentation
pose a heavy problem on nearly-filled-up pools made of
spinning HDDs (I believe so, at least judging from those
performance degradation problems writing to 80+%-filled
pools), and can fragmentation be effectively combatted
on ZFS at all (with or without BP rewrite)?

  For example, can(does?) metadata live "separately"
from data in some "dedicated" disk areas, while data
blocks are written as contiguously as they can?

  Many Windows defrag programs group files into several
"zones" on the disk based on their last-modify times, so
that old WORM files remain defragmented for a long time.
There are thus some empty areas reserved for new writes
as well as for moving newly discovered WORM files to
the WORM zones (free space permitting)...

  I wonder if this is viable with ZFS (COW and snapshots
involved) when BP-rewrites are implemented? Perhaps such
zoned defragmentation can be done based on block creation
date (TXG number) and the knowledge that some blocks in
certain order comprise at least one single file (maybe
more due to clones and dedup) ;)

What do you think? Thanks,
//Jim Klimov
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