The most overused sentence:
"It depends".
When I design Netapp solutions, I work
backwards from how much usable storage I'm going to need for the next 12-18
months. Then I also determine what kind of aggregate IOPS I'm going to
need from my disks.
From that, it's a fairly simple
calculation. OK, so it's not; I have a complicated excel spreadsheet that
I worked on for about a month before I was confident the answers were close
enough to use in designs; but before you start putting everything together
capacity wise you should have a good idea of how many disks you're going to need
to satisfy your performance requirements.
There's the obvious parity or dual-parity
overhead, and the hot spare (or multiple hot spares, depending on how many disks
you've got in the system).
There's the snapshot reserve for NAS
volumes (20% by default, you may need more or less)- but that reserve depends
highly upon the amount of changes and deletes you have in a given volume.
If you are using LUNs with snapshots, you need to multiple the size of the LUNs
by 2.2 (2x for the overwrite reserve and another 20% for the internal data
change rates inside a given lun, again change for your own
environment).
There's the WAFL RAID overhead; I've never
gotten a real good feel for that but let's call it 5%- (anyone care to amend
that?)
Now also keep in mind that WAFL volumes
don't really like to be more than 90% full because of the way they lay down
data; I like to keep mine at 80% or lower.
That's about all the overheads I can think
of. Sound like a lot? Any vendor with snapshot technology is going
to have the same issue of reserving loads of space for it, and they will use
raid-5, which requires a net loss of one disk every 5-8 disks (and be slow under
write load). Netapp goes 14 disks per parity (in either raid4 or
dp). And everyone with a hardware raid has some raid
overhead. Unless you're using raid 10, but then you're buying twice
the disk, aren't you?
Bottom line is that Netapp does require you
to invest some of your disk in data and physical availability. It's well
worth it, and it's comparable to every other enterprise system out there.
If all you need is dumb disk with no overhead and no avanced features, there are
plenty of RAID-0 solutions out there that will FC connect.
Glenn (the other one)
Hi all:
Can anyone please give me total NetApp overheads
including, file
systems, aggr reserve, snap reserve, wafl overhead, parity
disks
(raid_dp), etc, etc.
say, 10 x 100GB = 1000GB total.
what usable
space should i finally get after all those overheads.
Can anyone give me
proper figures/ math and proper break up of
above figure?
I heard
NetApp solution has lot of disk overheads!
Thank you in
advance.
Marge.
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