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

[Nir Soffer]

----- Original Message -----
> From: "Sven Kieske" <s.kieske at mittwald.de>
> To: devel at ovirt.org
> Sent: Thursday, October 23, 2014 4:31:02 PM
> Subject: [ovirt-devel] how (inefficient) does vdsm import sparse disks in local storage(file based) data domain?
> 
> 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.

We have this patch for next version:
http://gerrit.ovirt.org/33355

> 
> 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

Please do

> 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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