[ovirt-devel] how (inefficient) does vdsm import sparse disks in local storage(file based) data domain?

[Sven Kieske]

Hi,

I might be totally mistaken, so please tell me first if I understand
the process:

when vdsm imports a template from an export domain to local storage,
according to my "ps" output, vdsm copies sparse templates using "dd":

vdsm     14378  0.3  0.0  59032  4424 ?        S<l  14:43   0:05      \_
/usr/bin/qemu-img convert -t none -f raw
/rhev/data-center/d6ec1bb0-43ce-4e09-9a7a-e54f9779af64/4ed22092-e961-4e04-b77b-2648ad21f83a/image
vdsm     17291  0.0  0.0   8344   608 ?        D<   15:14   0:00      \_
/bin/dd iflag=direct
if=/rhev/data-center/mnt/$REDACTED:_home_iso/e7b8f2e2-b9f7-493c-96b5-33bfb091d0ba/dom_md/metadata
bs=4096 count=1

If this is correct I have some questions regarding this behaviour:

I understand that "dd" is "rock solid" works everywhere and for maybe
every storage/format combination and thus was maybe chosen to do the
work.

But if I'm right and I have a 100 GB sparse disk which actually
uses just 1 GB "dd" will copy the whole 100 GB over network, which
is really not efficient at all.

So here's my proposal (if I do not have any mistakes in the above):

at least make a:
tar -cpSvf disk.tar /path/to/disk
and than "dd" this over the network, because tar _can_ handle sparse
files.

Of course any other more efficient mechanism would be okay too.

If my observation is right, and the project think it's worth
to implement such a thing I would create an RFE for this
and I also would like to help code this (in my leisure time).

PS: I did observe this behaviour on ancient
ovirt 3.3 with vdsm version: vdsm-4.13.4-0.el6.x86_64
but I didn't see anything in release notes which
changed this behaviour.

-- 
Mit freundlichen Grüßen / Regards

Sven Kieske

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