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
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.
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