IBM N Series, Hardware Manual

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Chapter 14. Designing an N series solution
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In earlier versions on ONTAP, the aggregate had a default aggregate snapshot reserve of
5%. However, the modern administration tools (such as NSM) use a default of 0%. This
default often was used only in a MetroCluster or SyncMirror configuration. In all other
cases, it can safely be changed to 0%.
Effect of the WAFL file system
Another factor that affects capacity is imposed by the file system. The Write Anywhere File
Layou
t (
WAFL) file system that is used by the IBM System Storage N series has less effect
than many file systems, but the effect still exists. WAFL has a memory usage equal to 10% of
the formatted capacity of a drive. This memory is used to provide consistent performance as
the file system fills up. The reserved space increases the probability of the system locating
contiguous blocks on disk.
As a result, the example 2000 GB (decimal) disk drives are down to only slightly under 1500
GB (binary) before any user data is put on them. If you take the nine data drives per controller
and allocate them to a single large volume, the resulting capacity is approximately 13,400 GB
(binary), as shown in Figure 14-1.
Figure 14-1 Example of raw (decimal) to usable (binary) capacity
The example in Figure 14-1 is for a small system. The ratio of usable to raw capacity varies
depending on several factors, such as RAID group size, disk type, and space efficiency
features that can be applied later. Examples of these features include thin provisioning,
deduplication, compression, and Snapshot backup.
Effect of Snapshot protection
Consider the effect of Snapshot protection on capacity. Snapshot is a built-in capability that
keeps space free until it is used. However, the use of Snapshot affects the apparent usable
capacity of the storage system. It is common to run a storage system with 20% of space that
is reserved for Snapshot use. To the user, this space seems to be unavailable. The amount
that is allocated for this purpose can be easily adjusted when necessary to a lower or higher
value.
Running with this 20% setting further reduces the 13,400 GB usable storage to approximately
10,700 GB (binary). Whether you consider the snapshot reserve as being overhead or just
part of the usable capacity depends on your requirements.