Ahh, interesting information. Thanks folks, I'm have a better
understanding of this now.
--joe
Jeff Bonwick wrote:
is zfs any less efficient with just using a portion of a
disk versus the entire disk?
As others mentioned, if we're given a whole disk (i.e. no slice
is specified) then we can safely enable the write cache.
One other effect -- probably not huge -- is that the block placement
algorithm is most optimal for an outer-to-inner track diameter ratio
of about 2:1, which reflects typical platters. To quote the source:
http://cvs.opensolaris.org/source/xref/on/usr/src/uts/common/fs/zfs/metaslab.c#m
etaslab_weight
/*
* Modern disks have uniform bit density and constant angular velocity.
* Therefore, the outer recording zones are faster (higher bandwidth)
* than the inner zones by the ratio of outer to inner track diameter,
* which is typically around 2:1. We account for this by assigning
* higher weight to lower metaslabs (multiplier ranging from 2x to 1x).
* In effect, this means that we'll select the metaslab with the most
* free bandwidth rather than simply the one with the most free space.
*/
But like I said, the effect isn't huge -- the high-order bit that we
have a preference for low LBAs. It's a second-order optimization
to bias the allocation based on the maximum free bandwidth, which is
currently based on an assumption about physical disk construction.
In the future we'll do the smart thing and compute each metaslab's
allocation bias based on its actual observed bandwidth.
Jeff
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