ext3 zerofree option and RedHat back port?

Ulf Zimmermann ulf at openlane.com
Wed Sep 24 04:17:26 UTC 2008

> -----Original Message-----
> From: Eric Sandeen [mailto:sandeen at redhat.com]
> Sent: 09/23/2008 20:30
> To: Ulf Zimmermann
> Cc: Theodore Tso; ext3-users at 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
> > 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
> that it is!)
> But in any case the util itself is simple enough that building (or
> 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.


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