Re: Question about ext4 testing: need to produce a high depth extent tree to verify mapping code

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jul 05, 2019 at 11:49:02AM -0700, James Bottomley wrote:
> On Fri, 2019-07-05 at 10:39 -0700, Matthew Wilcox wrote:
> > On Fri, Jul 05, 2019 at 09:25:48AM -0700, James Bottomley wrote:
> > > Now the problem: I'd like to do some testing with high depth extent
> > > trees to make sure I got this right, but the files we load at boot
> > > are ~20MB in size and I'm having a hard time fragmenting the
> > > filesystem enough to produce a reasonable extent (I've basically
> > > only got to a two level tree with two entries at the top).  Is
> > > there an easy way of producing a high depth extent tree for a 20MB
> > > file?
> > 
> > Create a series of 4kB files numbered sequentially, each 4kB in size
> > until you fill the partition.  Delete the even numbered ones.  Create
> > a 20MB file.
> 
> Well, I know *how* to do it ... I was just hoping, in the interests of
> creative laziness, that someone else had produced a script for this
> before I had to ... particularly one which leaves more randomized gaps.

If you don't care about the contents of the file you could just build
src/punch-alternating.c from xfstests and use it to turn your 20M file
into holy cheese.

(Granted if you actually need 5,120 extents then you probably ought to
make it a 40M file and /then/ run it through the cheese grater....)

--D

> James
>