2011-07-15 11:10, phil.har...@gmail.com пишет:
If you clone zones from a golden image using ZFS cloning, you get fast, efficient dedup for free. Sparse root always was a horrible hack!
Sounds like a holy war is flaming up ;)

From what I heard, sparse root zones with shared common
system libraries allowed to save not only on disk space but
also on RAM. Can't vouch, never tested extensively myself.

Cloning of golden zones is of course used in our systems.
But this approach breaks badly upon any major systems
update (i.e. LiveUpgrade to a new release) - many of the
binaries change, and you either suddenly have the zones
(wanting to) consume many gigabytes of disk space which
are not there on a small rpool or a busy data pool, or you
have to make a new golden image, clone a new set of
zones and reinstall/migrate all applications and settings.

True, this is a no-brainer for zones running a single task
like an /opt/tomcat directory which can be tossed around
to any OS, but becomes tedious for software with many
packages and complicated settings, especially if (in some
extremity) it was homebrewn and/or home-compiled and
unpackaged ;)

I am not the first (or probably last) to write about inconvenience
of zone upgrades which loses the cloning benefit, and much
of the same is true for upgrading cloned/deduped VM golden
images as well, where the golden image is just some common
baseline OS but the clones all run different software. And it is
this different software which makes them useful and unique,
and too distinct to maintain a dozen of golden images efficiently
(i.e. there might be just 2 or 3 clones of each gold).

But in general, the problem is there - you either accept that
your OS images in effect won't be deduped, much or at all,
after some lifespan involving OS upgrades, or you don't
update them often (which may be inacceptable for security
and/or paranoia types of deployments), or you use some
trickery to update frequently and not lose much disk space,
such as automation of software and configs migration
from one clone (of old gold) to another clone (of new gold).

Dedup was a promising variant in this context, unless it
kills performance and/or stability... which was the subject
of this thread, with Edward's research into performance
of current dedup implementation (and perhaps some
baseline to test whether real improvements appear in
the future).

And in terms of performance there's some surprise in
Edward's findings regarding i.e. reads from the deduped
data. For infrequent maintenance (i.e. monthly upgrades)
zoneroots (OS image part) would be read-mostly and write
performance of dedup may not matter much. If the updates
must pour in often for whatever reason, then write and
delete performance of dedup may begin to matter.

Sorry about straying the discussion into zones - they,
their performance and coping with changes introduced
during lifetime (see OS upgrades), are one good example
for discussion of dedup, and its one application which
may be commonly useful on any server or workstation,
not only on on hardware built for dedicated storage.

Sparse-root vs. full-root zones, or disk images of VMs;
are they stuffed in one rpool or spread between rpool and
data pools - that detail is not actually the point of the thread.

Actual useability of dedup for savings and gains on these
tasks (preferably working also on low-mid-range boxes,
where adding a good enterprise SSD would double the
server cost - not only on those big good systems with
tens of GB of RAM), and hopefully simplifying the system
configuration and maintenance - that is indeed the point
in question.

//Jim

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