In general the blogs conclusion is correct . When file systems get full
there is
fragmentation (happens to all file systems) and for ZFS the pool uses gang
blocks of smaller blocks when there are insufficient large blocks.
However, the ZIL never allocates or uses gang blocks. It directly allocates
blocks (outside of the zio pipeline) using zio_alloc_zil() ->
metaslab_alloc().
Gang blocks are only used by the main pool when the pool transaction
group (txg) commit occurs. Solutions to the problem include:
- add a separate intent log
- add more top level devices (hopefully replicated)
- delete unused files/snapshots etc with in the poll...
Neil.
On 08/01/11 08:29, Josh Simon wrote:
Hello,
One of my coworkers was sent the following explanation from Oracle as
to why one of backup systems was conducting a scrub so slow. I figured
I would share it with the group.
http://wildness.espix.org/index.php?post/2011/06/09/ZFS-Fragmentation-issue-examining-the-ZIL
PS: Thought it was kind of odd that Oracle would direct us to a blog,
but the post is very thorough.
Thanks,
Josh Simon
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