RE: Some thoughts and questions on A-sis

>>3T for 3040 hmm.  I have an existing volume that holds 2.5T, so that is
pushing it
Why's that pushing it? sis should decrease the amt of used in that volume as
you've already seen by 26%, maybe more after a few more passes.

>>I wonder why the max size scales with the system model, I haven't thought
of a good reason for this yet since you could easily have lots of 3T
volumes.
It scales with model because higher model >> more RAM and faster processors.
Thus sis can address more storage. Yes, lots of 3T volumes, but that amt of
3T volumes will scale again by model platform...higher model, more RAM and
processor, hence can address more storage (higher amount of disks).

>>I was wondering if you can get sis to shrink say 2.8T to 2.0, is the 2.0
what counts for the sis volume limit size?
sis uses the size of the volume at time of turning sis on. If it's larger
than your model will permit, it doesn't turn on and you'd have to shrink it.
You can't grow the volume above the model limit if sis is turned on, you'd
have to turn it off to grow.

>>Even so, would I run into trouble if trying to do a full volume restore
and the full data set is over 3.0 but would have shrunk?
If you had to do a restore, it would be coming from either a sis'd volume
elsewhere (say it's Snapmirrored), or a sis aware tape backup environment
(could be mistaken there).

>>I noticed during my data copy, the snapshot reserve was blowing out big
time.
This is normal. As you copy data in fresh, the snapshots (your backups) will
put pointers to the blocks that have changed (the new data flowing in), All
the data in essence has changed. If you're copying fresh data to the
storage, may as well turn snapshots off until all the data is copied in.
It won't be sis and snapshots contending here for same blocks...sis and
snapshot's won't paper-rock-scissors for the same blocks as they both, in
essence, treat blocks the same way (sis is highly based on snapshot
foundation when you break it down to the 4k block level).

hth,

Best regards,
~~~~~~~~~~~~~~~~~~~~~
Kevin Parker
http://theparkerz.com
~~~~~~~~~~~~~~~~~~~~~


-----Original Message-----
From: owner-toasters@... [mailto:owner-toasters@...] On
Behalf Of Adam McDougall
Sent: Wednesday, March 26, 2008 10:08 PM
To: toasters@...
Subject: Some thoughts and questions on A-sis

I started testing sis on one of our 3040's last weekend.  Sorry if theres
more extensive information on NOW, but the things I've found so far were
fairly rudimentry.  Some impressions and questions:

I found out quickly that my 5T test volume was too big.  Looked up the
limits because I had no idea there were volume size limits.  3T for 3040
hmm.  I have an existing volume that holds 2.5T, so that is pushing it, and
while I would like to split it up, it won't happen overnight.  I'm not
desperate to use sis on it, although I am almost done copying it and so far
have realized 26% savings from dedupe for testing.  I wonder why the max
size scales with the system model, I haven't thought of a good reason for
this yet since you could easily have lots of 3T volumes.  I was wondering if
you can get sis to shrink say 2.8T to 2.0, is the 2.0 what counts for the
sis volume limit size?  Even so, would I run into trouble if trying to do a
full volume restore and the full data set is over 3.0 but would have shrunk?
Once I write over 3T into a volume, it sounds like I cannot run sis on it.
I'm not too worried about having to do a full restore of a volume from tape,
but I don't want to limit my future options if the short term payoff isn't
worth it.  I'm pretty sure I'll use it on all or most of all of my other
volumes since they are much smaller.

I noticed during my data copy, the snapshot reserve was blowing out big
time.  It seemed like almost any savings from sis went into snapshots for
some reason.  100, 200, 400% full, and it started trimming itself back after
a while.  Since this was a non-production copy, I didn't care, and deleted
those snapshots this morning, but I thought that was rather odd, and
couldn't think what the snapshots would have to offer since if the data was
unmodified at the filesystem level, the snapshot should contain the same
data.  While not a problem for an initial copy, I would expect the same
thing to happen when sis runs during production, and although it wouldn't be
as large and would eventually flush out, why am I expending space to store
duplicate copies of data that I asked it to deduplicate? :) Maybe its just
because wafl identifies them as "changed blocks" and insists on storing them
in the snapshot.

A 1 gig file of zeros still takes several tens of megabytes after sis.  
hmm :) And roughly twice as much for a second copy of it.