than *zpool list*.
So try z*pool list*.
Thank you for the explanations. Really useful. Looking over the current
of zfs list. Is that something that is present only at Delphix at the
Regards.
You can use it on existing pools but the histogram only gets created when
you condense a space_map (a process by which ZFS tries to make the
space_map ondisk smaller). This means that when you look at an existing
pool it may have some space_maps with histograms and ones without.
Thanks,
George
Wow!
Thank you very much George, you are both a gentlemen and a scholar to an
order of magnitude!
I assume it best to implement on fresh pools to see the most benefits?
My path will most limey be to replicate on a system with it enabled, then
switch over to that system. Iâve 2 identical servers for one of our main
production resources.
- aurf
"Janitorial Services"
The space_map histogram feature has been used in production at Delphix
for a while. The data it provide is being used to make smarter decisions
about how and where to allocate blocks. It also gives us an idea of how
badly fragmented things get on a pool. For example, we've come up with a
simplistic metric to define pool fragmentation based on the histogram
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH
ALTROOT
dcenter 8.22T 5.50T 2.71T - 53% 66% 1.00x ONLINE -
The higher the frag% the more your free space is comprised of small
segments. We have been making many changes in this area and more to come
but the idea behind the histogram was to be able to lay the foundation for
improvements in allocation strategies. You can take a closer look at the
histogram on your pool by running 'zdb -mm[m] <pool>'.
Hope this helps. I will try to get a more extensive write up in a blog
along with some of the perf enhancements we've seen by making smarter
allocations decisions. Unfortunately right now I'm pretty busy with
compressed ARC. If you have more questions please feel free to ask (post or
email directly).
Thanks,
George
By the way, any one know who George is?
Iâd like to ping him and see if he is satisfied yet ? :)
- aurf
"Janitorial Services"
Hi Steven,
I gleaned this from an older Illumos post found here;
http://comments.gmane.org/gmane.os.illumos.zfs/2565
Of interest;
- There is a new spacemap_histogram on-disk feature flag. When it is
enabled spacemaps store more data about the amount of contiguous free
space in metaslabs. The current disk format only stores the total
amount of free space, which means that heavily fragmented metaslabs
can look appealing, causing us to read them off disk, even though they
don't have enough contiguous free space to satisfy large allocations,
leading us to continually load the same fragmented space maps over and
over again. The allocation algorithm that uses this information is
disabled by default and can be enabled via a tunable. It will become
the default allocator once George is satisfied with the amount of
performance testing it has received. We have extensively tested this
code bo
t
h with the t
unable enabled and disabled. It is not possible to
seperate out spacemap_histogram from the rest of this refactoring
because they were done together.
- aurf
"Janitorial Services"
I've not used it I'm afraid as the man page provides no
real indication of what its benefit would be if used.
"This features allows ZFS to maintain more information
about how free space is organized within the pool.
Regards
Steve
----- Original Message ----- From: "aurfalien via illumos-zfs" <
Sent: Wednesday, September 10, 2014 3:20 PM
Subject: [zfs] spacemap_histogram
Hi,
Iâve asked on the FreeBSD lists but no takers.
Curious if any one has tried the spacemap_histogram feature in production.
Seems cool, in theory anyways.
Thanks in advance,
- aurf
"Janitorial Services"
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