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Re: ext3 zerofree option and RedHat back port?



Ulf Zimmermann wrote:
-----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.



Note that there is work on getting file systems to use the new TRIM (for S-ATA drives) and its equivalent proposed standard in T10 SCSI for arrays which will give you this automatically. David Woodhouse was pushing patches for TRIM, we are still thinking about the SCSI versions...

ric


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