10% overhead for WAFL, FYI.
As to what Glenn says, I agree –
start with storage space needed, then figure performance. Don’t fill the
remaining volume beyond 90% if possible (unless it’s all archive-type
data).
As for your question before:
If you use RaidDP, you’ll lose 2
disks for every raid group (typically 16 disks total, or 14D+2P)
The disks are right-sized (to ensure that
every manufacturer has disks with the same size, start to end – ie,
You’re left with # data disks * disk
size, MINUS 10% WAFL overhead.
After that, factor in snapshot reserve
(20% by default) and space reservations if needed.
In the case of 320GB ATA disks, the right
size is actually 274400 MB (the 320GB is a raw or unformatted capacity –
you can thank the ATA drive manufacturers for that misleading info). A RG of
size 16 would net you 3457440MB after 10% overhead. If you don’t need
snapshots, that’s your usable space – if you do need snapshot
reserve, just subtract that from the total.
Using LUNs (especially with snapdrive)
changes the rules because of the 2x overhead it enforces. This is actually
changed in with something called fractional reserve (snapdrive supports
fractional reserve with SD 4.0 and ONTAP 7.1)
The reason for all of the overhead: data
protection.
Netapp doesn’t sell space –
they sell insurance. They sell protection from data loss FIRST, then
performance\manageability, and lastly space.
I would say that NetApp has no more
overhead than any other vendor that has snapshot like capabilities (and
probably less given that everyone else is a copy-on-write implementation
thusfar).
Glenn
From:
owner-toasters@mathworks.com [mailto:owner-toasters@mathworks.com] On Behalf Of Glenn Dekhayser
Sent: Wednesday, July 26, 2006
11:11 PM
To: margesimpson@hushmail.com
Cc: toasters@mathworks.com
Subject: RE: Storage space
overheads!
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)
From:
owner-toasters@mathworks.com on behalf of margesimpson@hushmail.com
Sent: Wed 7/26/2006 9:38 PM
To: toasters@mathworks.com
Subject: Storage space overheads!
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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