This is the first in a series of posts about how space allocation works in Data ONTAP. Because the topic is lengthy, I split the concepts up into multiple, easier to digest posts.
Space allocation in NetApp’s Data ONTAP is a question that comes up fairly often because it’s a concept that operates in three dimensions.
Physical – How many actual disks do we have?
Virtual – How much space have we told the software to allocate via FlexVols?
Actual – How much space are we actually using?
Then when you throw in things like snapshots, space guarantees, LUNs… it starts to get a bit muddled.
But before we get into how all that works, let’s talk about why it matters.
Why do I care about space allocation?
If you’re not monitoring your space in some capacity (pun intended), you’re doing it wrong. For Data ONTAP, NetApp already provides some space reporting utilities in the OnCommand, particularly rich with OnCommand Insight.
As a storage administrator, you have to ensure you know how much space you are using. If you run out of space, you can see issues with performance, write failures, backup or snapshot failures, or LUNs going offline.
Basically, if you ignore how much space you have, you are creating a potential outage scenario on your production storage. If you ignored the space problem for an indefinite amount of time, you may also have painted yourself into a corner because you didn’t have enough space to take backups.
In Data ONTAP, if you let your capacity approach 85-95%, you may start to see performance issues depending on your workload. Because WAFL is a filesystem that can write to any free block in storage, as the file system fills, it can take longer to find a free block to write to in some instances.
Space issues are not a concept that is unique to Data ONTAP – every storage vendor has a very similar set of storage capacity considerations that need to be monitored and addressed.
Lost in Space Table of Contents
This post covers how much space you actually start out with when you buy disks.
Stay tuned for more posts including:
Part 3: Snapshots and snapshot reservations
Part 4: Physical space allocation vs. Virtual space allocation
Part 5: Flexible Volumes, Space Guarantees and Space Allocation for NAS and SAN
Part 6: Space management in Data ONTAP