Can l2arc cache be bypassed?

[Carlton1975]

I've got a freenas (9.3) with 2x 100gb ZIL ssd's, 8*2tb sas disks and 1x 480GB l2arc cache. The freenas is providing iscsi storage for 3 servers. I've started thinking about backups and l2arc cache. The nas is still in testing mode, and not yet used in production.

We are using veeam to replicate our servers (mostly web and database servers) offsite, I assume, that when I'm creating backups with, the data which i'm sending over will be stored in the l2arc cache. (freenas just see's iscsi traffic going towards the esxi hosts, and does not know that the traffic is backup related).

To my mind the l2arc cache should hold the most important data such as website content and sql/mysql data. I understand the data that is inside the cache cannot be controlled as it works on a block level, it simply does not see the difference between a static file on a iscsi volume or a database.

Would it be possible to bypass the l2arc function for backup related tasks?

What do you guys think about this? Are there ways around it or will the l2arc cache always be used? ... Is it even worth thinking about?

 

[jgreco]

It can't really be bypassed, no, but the ARC system is reasonably intelligent and doesn't just cache data because it was accessed once. It might well push a block that was accessed just once out to L2ARC if there's free space. but otherwise it is going to prefer stuff that's most in demand. As long as the L2ARC is serving up data that results in a lowering of the load on the pool, that's a win for pool performance. ZFS may not always do that as precisely optimally as possible, but the ARC algorithm hasn't been shown to be totally unsuitable or broken. ZFS will focus on keeping in ARC the stuff that is most used, and the L2ARC gets the stuff that is being pushed out of ARC, so that'll generally be the stuff that is not-quite-most-used.

Don't worry about it. Let the system figure it out. Go out and have a beer while Veeam backs up your stuff. We live in an age of powerfully near-magic software.

 

[Carlton1975]

Thanks for your answer jgreco, great clarification!! I might try to optimize it to much :P