On 08/01/2010 14:50, David Dyer-Bennet wrote:
On Fri, January 8, 2010 07:51, Robert Milkowski wrote:
On 08/01/2010 12:40, Peter van Gemert wrote:
By having a snapshot you
are not releasing the
space forcing zfs to allocate new space from other
parts of a disk
drive. This may lead (depending on workload) to more
fragmentation, less
localized data (more and longer seeks).
ZFS uses COW (copy on write) during writes. This means that it first has
to find a new location for the data and when this data is written, the
original block is released. When using snapshots, the original block is
not released.
I don't think the use of snapshots will alter the way data is fragmented
or localized on disk.
Well, it will (depending on workload).
For example - lets say you have a 80GB disk drive as a pool with a
single db file which is 1GB in size.
Now no snapshots are created and you constantly are modyfing logical
blocks in the file. As ZFS will release the old block and will re-use it
later on so all current data should be roughly within the first 2GB of
the disk drive therefore highly localized.
I thought block re-use was delayed to allow for TXG rollback, though?
They'll certainly get reused eventually, but I think they get reused later
rather than sooner.
yes there is a delay but iirc it is only several transactions while the
above scenario in practice usually means a snapshot a day and keep 30 of
them.
--
Robert Milkowski
http://milek.blogspot.com
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