Re: Any experience using "shake" defragmenter?

Mladen Gogala wrote:
> Did anyone try using "shake" while the cluster is active? Any problems
> with corruption or data loss? I ran the thing on my home directory and
> nothing was broken. I didn't develop any performance test, so cannot
> vouch for the effectiveness of the procedure. Did anyone play with
> that? Any positive or negative things to say about shake?
>

Shake works by allocating a new file the size of the original, in what
is presumed to be then be unfragmented space. It copies the original
over to this new space and then gets rid of the original. That
procedure will cause database corruption if the server happens to access
the file it's moving while it's in the middle of doing so. If the
database isn't running, though, it is probably fine.

On ext3 you can measure whether it was useful or not by taking the
filesystem off-line and running fsck before/after using it. Look for
percentages given for "non-contiguous files" and directories. If those
were low to begin with, little reason to run the utility. If they're
high, running shake should bring them down afterwards if it's doing its
job right.

On a PostgreSQL database system, you can get the same basic effect while
leaving the server up--but just with the table locked--using CLUSTER.
And that will clean up a bunch of other potential messes inside the
database that shake can't touch. I just do that instead if I'm worried
a particular table has become fragmented on disk.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books