sparsify - utility to punch out blocks of 0s in a file
sunil.mushran at oracle.com
Mon Feb 6 12:40:11 CST 2012
On 02/04/2012 12:04 PM, Eric Sandeen wrote:
> Now that ext4, xfs,& ocfs2 can support punch hole, a tool to
> "re-sparsify" a file by punching out ranges of 0s might be in order.
> I whipped this up fast, it probably has bugs& off-by-ones but thought
> I'd send it out. It's not terribly efficient doing 4k reads by default
> I suppose.
> I'll see if util-linux wants it after it gets beat into shape.
> (or did a tool like this already exist and I missed it?)
> (Another mode which does a file copy, possibly from stdin
> might be good, like e2fsprogs/contrib/make-sparse.c ? Although
> that can be hacked up with cp already).
> It works like this:
> [root at inode sparsify]# ./sparsify -h
> Usage: sparsify [-m min hole size] [-o offset] [-l length] filename
So I have a similar tool queued up in ocfs2-tools. Named puncher.
I'll pull it out if we get something in util-linux. But maybe you can
extract something useful from it.
Like.... maybe doing dry-run as default. It is an inplace modification
after all. Also using a large hole size as default (1MB). Over using
hole punching will negatively affect read performance. We should make
the sane choice for the user.
On a related note, it may make sense for ext4 to populate the cluster
size (bigalloc) in stat.st_blksize.
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