Andrew Snow andrew at modulus.org
Tue Sep 30 01:41:47 UTC 2008

Zaphod Beeblebrox wrote:
>  Also, there
> exists data within the ARC (I'm always tempted to say the ARC Cache, but 
> that is redundant) that is also then in paging memory.

OK, but one advantage of ZFS memory consumption is under heavy write
loads, where much of the memory is used to store and reorder writes.
The heavy memory consumption under reading is a shame, but ZFS has to
cache and use more metadata than UFS, so its a price you pay for the
extra features and benefits.

What I think we need is a way to turn off read-caching except for
metadata.  This allows ARC to only be used more efficiently.  Currently
you can turn all read-ahead on or off, with the provided sysctl
tunables, but would be easy to implement a metadata-only option.  I
found that access speed suffers when metadata is not prefetched.

If you are running an X workstation with 2GB or less memory, then I
agree ZFS is a bad default choice.

For my workstation I would still use ZFS, I would:

* turn down ARC size,

* turn off read-ahead except for metadata,

* and even turn off ZIL and write cache flushing, which solves the
annoyance of unpredictable delays when flushing buffers.   Not a good
choice for a server but perfect for a workstation.

- Andrew

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