[zfs-discuss] ZFS eating up all 16 GB RAM when combining 4k blocksize ZVOL with XFS

Omar Siam simar at gmx.net
Tue Nov 13 10:22:19 EST 2018


Hi Paul!

> I would recommend using the 0.7.11 dkms versions of zfs and spl. Too 
> many new features and setups in there.
I'm sure the Ubuntu team backports any critical fix. Just the enterprise 
Linux thing of never raise the version number. But I agree with you 
there were some important fixes between 0.7.5 and 0.7.11.
>
> The ARC *will* take up at least half your RAM if you do not limit it 
> in your grub.cfg moreso if youve got zraid setups going on.
Well I want that to happen. In this setup the only thing that should use 
RAM are ZFS and XFS or SCST for iSCSI. But I am still puzzled that this 
setup can use more than double the configured ARC size in a matter of 
seconds.
>
> As for XFS - Im not sure whats happening there. Shouldnt be anything 
> too crazy.
>
> On Mon, Nov 12, 2018, 18:07 Paul Gorman <pagorman at asu.edu 
> <mailto:pagorman at asu.edu> wrote:
>
>     Add to the grub/rEfind boot args for the kernel:
>
>     zfs.zfs_arc_max=536870912 #(for 512MB cap zfs memory usage,
>     expressed in bytes)
>
>     Though you'll likely want 512MiB-1GiB per physical disk.
>
Yes with this low setting it works. I would really like to use more of 
the RAM as ARC.
If possible the ZFS/SPL code should not just use all available free RAM 
and then some more out of the blue. Seems at least there has to be a 
different default value than half the RAM if there are reproducable 
cases where that just more than doubles. No user expects the filesystem 
code to just exhaust all memory. Swap changes nothing in this respect by 
the way.

Best regards

Omar
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