-----Original Message-----
From: Eric Sandeen [mailto:sandeen redhat com]
Sent: 09/23/2008 20:30
To: Ulf Zimmermann
Cc: Theodore Tso; ext3-users redhat com
Subject: Re: ext3 zerofree option and RedHat back port?
Ulf Zimmermann wrote:
Reason I asked is this. We use currently 3Par S400 and E200 as SAN
arrays. The new T400 and T800 has a built in chip to do more
intelligent
thin provisioning but I believe even the S400 and E200 we have will
free
on the SAN level a block of a thin provisioned volume if it gets
zero'ed
out. Haven't gotten around yet to test it, but I am planning on. We
are
currently using 3 different file system types, one is a propriety
from
Onstor for their Bobcats (NFS/CIFS heads) where I believe I have
observed just freeing of SAN level blocks. The two other are EXT3
and
OCFS2.
Ok, so you really want to zero the unused blocks in-place, and e2image
writing out a new sparsified image isn't a ton of help.
The tool does that, I guess - but only on an unmounted or RO-mounted
filesystem, right? (plus I'd triple-check that it's doing things
correctly, opening a block device and splatting zeros around, one
hopes
that it is!)
But in any case the util itself is simple enough that building (or
even
packaging) for fedora/EPEL should be trivial.
(FWIW, there is work upstream for filesystems to actually communicate
freed blocks to the underlying storage, just for this purpose...)
-Eric
I am going to try it out by hand. Create a thin provisioned volume,
write random crap to it, then zero the blocks. See if that shrinks the
physical allocated space.
Ulf.