Re: BBU Cache vs. spindles

Jesper Krogh wrote:
> Can you point to some ZFS docs that tell that this is the case..  I'd
> be surprised
> if it doesnt copy away the old block and replaces it with the new one
> in-place. The
> other behaviour would quite quickly lead to a hugely fragmented
> filesystem that
> performs next to useless and ZFS doesnt seem to be in that category..

http://all-unix.blogspot.com/2007/03/zfs-cow-and-relate-features.html

"Blocks containing active data are never overwritten in place; instead,
a new block is allocated, modified data is written to it, and then any
metadata blocks referencing it are similarly read, reallocated, and
written."

http://opensolaris.org/jive/thread.jspa?messageID=19264 discusses how
this interacts with the common types of hardware around:  no guaratees
with lying hard drives as always, but otherwise you're fine.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
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