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Date: Sat, 2 Feb 2008 12:52:25 +0100 (CET)
From: Jan Engelhardt <jengelh@...putergmbh.de>
To: Pavel Machek <pavel@....cz>
cc: Laurent Vivier <Laurent.Vivier@...l.net>,
Paul.Clements@...eleye.com, nbd-general@...ts.sourceforge.net,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Allow NBD to be used locally
On Feb 2 2008 12:23, Pavel Machek wrote:
>On Fri 2008-02-01 14:25:32, Laurent Vivier wrote:
>> This patch allows Network Block Device to be mounted locally.
>
>What is local nbd good for? Use loop instead...
Local NBD is good for when the content you want to make available
through the block device is dynamic (generated on-the-fly),
non-linear or supersparse.
Take for example VMware virtual disks. Just a guess, but
they roughly can look like this:
kilobytes 0.. 1: header
kilobytes 1..10: correspond to LBA 0..20
kilobytes 11..20: correspond to LBA 40..60
kilobytes 21..22: correspond to LBA 22..23
So what we have is non-linearity -- LBA 22 comes after LBA 40 -- loop
does not deal with that.
And there is supersparsity -- the VMDK file itself is complete, but
unallocated regions like LBA 24..40 are sparse/zero when projected
onto a file/block device, respectively; loop cannot deal with that
either.
In fact, VMware uses local nbd today for its vmware-loop helper
utility, most likely because of the above-mentioned reasons. (Though
it quite often hung last time I tried.)
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