On Mon, 16 Jan 2012, Jim Klimov wrote:


I think that in order to create a truly fragmented ZFS layout,
Edward needs to do sync writes (without a ZIL?) so that every
block and its metadata go to disk (coalesced as they may be)
and no two blocks of the file would be sequenced on disk together.
Although creating snapshots should give that effect...

Creating snapshots does not in itself cause fragmentation since COW would cause that level of fragmentation to exist anyway. However, snapshots cause old blocks to be maintained so the disk becomes more full, fresh blocks may be less appropriately situated, and the disk seeks may become more expensive due to needing to seek over more tracks.

In my experience, most files on Unix systems are re-written from scatch. For example, when one edits a file in an editor, the editor loads the file into memory, performs the edit, and then writes out the whole file. Given sufficient free disk space, these files are unlikely to be fragmented.

The case of slowly written log files or random-access databases are the worse cases for causing fragmentation.

Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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