On Mon, 19 Apr 2010, Don wrote:
Continuing on the best practices theme- how big should the ZIL slog disk be?
The ZFS evil tuning guide suggests enough space for 10 seconds of my
synchronous write load- even assuming I could cram 20 gigabits/sec
into the host (2 10 gigE NICs) That only comes out to 200 Gigabits
which = 25 Gigabytes.
Note that large writes bypass the dedicated intent log device entirely
and go directly to a ZIL on primary disk. This is because SSDs
typically have much less raw bandwidth than primary disk does. If you
are writing bulk data, the larger chunks will go directly to primary
disk, and the smaller bits (e.g. smaller writes, metadata, filenames,
directories, etc.) will go to the dedicated intent log device. This
means that the device does not need to be as large as you may think it
should be.
Use the 'zilstat' DTrace script to evalutate what really happens on
your system before you invest in extra hardware.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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