ZFS: Disabling ARC?

[Alexander Leidinger]

Quoting Matt Churchyard <matt.churchyard at userve.net> (from Mon, 3 Aug  
2015 15:00:07 +0000):

> I don't know if it's changed, but even changing ARC to metadata only  
> used to cause massive performance issues when reading large files in  
> small chunks. Reading a 128k ZFS record in 4k chunks would cause ZFS  
> to read the same 128k record from disk 32 times.
> There's a forum thread about it here -  
> https://forums.freebsd.org/threads/zfs-primarycache-all-versus-metadata.45555/

When using "metadata" as the setting, you tell that you are OK to  
trade in some (data access, not metadata-access) performance for  
memory. Determining if the resulting performance is acceptable is up  
to you. If you blindly set the value without validating that it is OK  
for you, you shall not complain (= "measure, change, measure" and not  
"measure, change, hope"). I agree that not for all changes the  
resulting effects are 100% obvious and explicitely stating the  
edge-cases is appreciated. Feel free to add some words about it to  
https://wiki.freebsd.org/ZFSTuningGuide

Bye,
Alexander.
-- 
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http://www.FreeBSD.org    netchild at FreeBSD.org  : PGP 0xC773696B3BAC17DC