When to Add the First Read Cache Device (CLI)

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Use the following procedure to determine if you need a first read cache device for the appliance. To determine if you need more than one device, see When to Add More Read Cache Devices (CLI).

Create a worksheet as described in Creating a Worksheet (CLI), select that worksheet, and then enter dataset.
```
hostname:analytics worksheets> select worksheet-000
hostname:analytics worksheet-000> dataset
```

Enter set name=arc.accesses[hit/miss], and then enter commit to add cache ARC access broken down by hit/miss to your worksheet.

hostname:analytics worksheet-000 dataset (uncommitted)> set name=arc.accesses[hit/miss]
                           name = arc.accesses[hit/miss]
hostname:analytics worksheet-000 dataset (uncommitted)> commit

Enter dataset.

hostname:analytics worksheet-000> dataset

Repeat steps 2 and 3 to add the following datasets:

Cache ARC accesses broken down by metadata hits and misses ("arc.accesses[hit/miss=metadata hits][L2ARC eligibility]")
Cache ARC accesses broken down by data hits and misses ("arc.accesses[hit/miss=data hits][L2ARC eligibility]")

hostname:analytics worksheet-000 dataset (uncommitted)> set name="arc.accesses[hit/miss=metadata hits][L2ARC eligibility]"
                           name = arc.accesses[hit/miss=metadata hits][L2ARC eligibility]
hostname:analytics worksheet-000 dataset (uncommitted)> commit
hostname:analytics worksheet-000> dataset
hostname:analytics worksheet-000 dataset (uncommitted)> set name="arc.accesses[hit/miss=data hits][L2ARC eligibility]"
                           name = arc.accesses[hit/miss=data hits][L2ARC eligibility]
hostname:analytics worksheet-000 dataset (uncommitted)> commit

Enter done, and then enter done again to exit the context.

hostname:analytics worksheet-000> done
hostname:analytics worksheets> done

Wait several minutes, and then go to analytics datasets.
Note - This wait time can be adjusted to better determine your peak I/O. Capturing analytics for 24 hours during normal business operations may reveal the best understanding of your IO patterns.
```
hostname:> analytics datasets
```

Enter show to view a list of available datasets.

hostname:analytics datasets> show
Datasets:

DATASET     STATE    INCORE  ONDISK  NAME
dataset-000 active    1.27M   15.5M  arc.accesses[hit/miss]
dataset-001 active     517K   9.21M  arc.accesses[hit/miss=metadata hits][L2ARC eligibility]
...
hostname:analytics datasets>

Enter select and the dataset with the name arc.accesses[hit/miss=metadata hits][L2ARC eligibility].
In this example, dataset name arc.accesses[hit/miss=metadata hits][L2ARC eligibility] corresponds to dataset-001.
```
hostname:analytics datasets> select dataset-001
```
Enter read 86400 to read the last 86,400 seconds, or 24 hours, of the dataset. When you are finished examining the data, enter done.
```
hostname:analytics dataset-001> read 86400
...
hostname:analytics dataset-001> done
```
Consider adding the first read cache device when all of the following conditions are present:
- There are at least 1500 L2ARC-eligible ARC access misses for metadata per second
- The appliance has an active filesystem or LUN with a ZFS recordsize of 32k or smaller

Enter show to view a list of available datasets.

hostname:analytics datasets> show
Datasets:

DATASET     STATE    INCORE  ONDISK  NAME
dataset-000 active    1.27M   15.5M  arc.accesses[hit/miss]
dataset-001 active     517K   9.21M  arc.accesses[hit/miss=metadata hits][L2ARC eligibility]
dataset-002 active     780K   9.20M  arc.accesses[hit/miss=data hits][L2ARC eligibility]
...
hostname:analytics datasets>

Enter select and the dataset with the name arc.accesses[hit/miss=data hits][L2ARC eligibility].
In this example, dataset name arc.accesses[hit/miss=data hits][L2ARC eligibility] corresponds to dataset-002.
```
hostname:analytics datasets> select dataset-002
```
Enter read 86400 to read the last 86,400 seconds, or 24 hours, of the dataset. When you are finished examining the data, enter done.
```
hostname:analytics dataset-002> read 86400
...
hostname:analytics dataset-002> done
```
Consider adding the first read cache device when all of the following conditions are present:
- There are at least 1500 L2ARC-eligible ARC access misses for data per second
- The appliance has an active filesystem or LUN with a ZFS recordsize of 32k or smaller