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Old 01-06-2010, 08:09 PM
Andreas Dilger
Default Optimizing dd images of ext3 partitions: Only copy blocks in use by fs

On 2010-01-06, at 04:00, Martin Baum wrote:
for bare-metal recovery I need to create complete disk images of
ext3 partitions of about 30 servers. I'm doing this by creating lvm2-
snapshots and then dd'ing the snapshot-device to my backup media. (I
am aware that backups created by this procedure are the equivalent
of hitting the power switch at the time the snapshot was taken.)

This works great and avoids a lot of seeks on highly utilized file
systems. However it wastes a lot of space for disks with nearly
empty filesystems.

It would be a lot better if I could only read the blocks from raw
disk that are really in use by ext3 (the rest could be sparse in the
imagefile created). Is there a way to do this?

You can use "dump" which will read only the in-use blocks, but it
doesn't create a full disk image.

The other trick that I've used for similar situations is to write a
file of all zeroes to the filesystem until it is full (e.g. dd if=/dev/
zero of=/foo) and then the backup will be able to compress quite
well. If the filesystem is in use, you should stop before the
filesystem is completely full, and also unlink the file right after it
is created, so in case of trouble the file will automatically be
unlinked (even after a crash).

I am aware that e2image -r dumps all metadata. Is there a tool that
does not only dump metadata but also the data blocks? (maybe even in
a way that avoids seeks by compiling a list of blocks first and then
reading them in disk-order) If not: Is there a tool I can extend to
do so / can you point me into the righ direction?

(I tried dumpfs, however it dumps inodes on a per-directory base.
Skimming through the source I did not see any optimization regarding
seeks. So on highly populated filesystems dumpfs still is slower
than full images with dd for me.)

Optimizing dump to e.g. sort inodes might help the performance, if
that isn't already done.

Cheers, Andreas
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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