On Sat, 31 Jul 2010, Gregory Gee wrote:
The maximum size of a log device should be approximately 1/2 the
size of physical memory because that is the maximum amount of
potential in-play data that can be stored. For example, if a system
has 16 Gbytes of physical memory, consider a maximum log device size
of 8 Gbytes.
This sounds like misleading bad advice to me, even if it is
technically correct. Only synchronous writes go into the slog.
Typical sources of synchronous writes are databases and NFS service.
In the case of NFS, the maximum amount of synchronous writes per
transaction group (TXG) is limited by wire bandwidth and other
latencies.
The 'zilstat' dtrace script can be used to determine the actual
synchronous write load on a running server.
*********
I don't think I can get SSD that small. It seems like a waste of
larger SSD now. Is there a way to make better use of the SDD by
partitioning? I have two pools, could I partition the SSD to have
each partition as a ZIL for the different pools? Any other creative
uses?
FLASH-based SSDs wear out due to repeated writes. The formatted size
of the slog becomes immaterial as long as it is large enough for the
synchronous writes assigned to a TXG. An over-provisioned SSD is
going to last much longer under heavy write load than a
perfectly-sized one.
For a small server, a reasonable approach is to partition the SSD so
that it satisfies both slog and l2arc requirements. The l2arc
requirement is primarily read access so it does not tend to wear the
SSD out as quickly as use as an slog. A well designed SSD will
include device-wide wear leveling so that seldom used FLASH blocks are
reassigned to replace heavily-used FLASH blocks. This means that
blocks from the lightly-used partition can be re-assigned to replace
blocks from the heavily-used partition.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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