Cache drive

[Halfe]

hi.

Been Reading a bit. trying to understand the benefit of a cache drive.

My system build. (in my signature)

Im having a lot of small time read on my NAS and i want to take some load of the Drives.
Does the cache drive help me in that it takes away some of the stress of read/write on the normal drives
i'm planning on using a 128GB SSD drive.

 

[jgreco]

No.

1) Your ARC to L2ARC ratio should be about 1:5, so your largest recommended L2ARC would only be about 40GB.

2) ZFS requires a lot of ARC in order to do a good job identifying the best blocks to evict to L2ARC. We usually find this to be at least 32-64GB of RAM.

3) So ZFS will instead evict questionably useful data to the L2ARC, but will require space in the ARC to maintain those L2ARC pointers. This means that you are artificially reducing ARC for little benefit.

4) L2ARC isn't really meant to "take away some of the stress" on drives. What it is meant to address is to improve responsiveness on a busy pool. If your pool is not busy, you generally will not notice much (or any) benefit from L2ARC.

So you'll most likely just slow your system down further by adding L2ARC.

 

[Halfe]

so what can i do about this activity?? its the same on all my drives

 

[jgreco]

What do you mean, "do about this activity"? You mean, like, you can sit there and watch the lights blink occasionally? Do you object to the NAS doing its NASly duties and keeping those very graphs you're quoting?

 

[Robert Trevellyan]

Im having a lot of small time read on my NAS

Those graphs actually show a lot of small time write activity, which is normal for FreeNAS.

 

[joeschmuck]

@Halfe This is normal activity, I have the same thing, just over 100k write on average when it's just sitting idle.

 

[Halfe]

Ok. But i have a "JBOD" consisting of 10 500GB drives and 2 2TB drives. Only the 500 drives witch are in Z2 have this activity. And the 2TB drives are not.



Sent fra min SM-N910F via Tapatalk

 

[jgreco]

Correct, because you probably have the system dataset on the RAIDZ2 that has that pool.