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From: | Eric Blake |
Subject: | Re: Cross-project NBD extension proposal: NBD_INFO_INIT_STATE |
Date: | Wed, 12 Feb 2020 06:09:11 -0600 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
On 2/12/20 1:27 AM, Wouter Verhelst wrote:
Hi, On Mon, Feb 10, 2020 at 10:52:55PM +0000, Richard W.M. Jones wrote:But anyway ... could a flag indicating that the whole image is sparse be useful, either as well as NBD_INIT_SPARSE or instead of it? You could use it to avoid an initial disk trim, which is something that mke2fs does:Yeah, I think that could definitely be useful. I honestly can't see a use for NBD_INIT_SPARSE as defined in this proposal; and I don't think it's generally useful to have a feature if we can't think of a use case for it (that creates added complexity for no benefit). If we can find a reasonable use case for NBD_INIT_SPARSE as defined in this proposal, then just add a third bit (NBD_INIT_ALL_SPARSE or something) that says "the whole image is sparse". Otherwise, I think we should redefine NBD_INIT_SPARSE to say that.
Okay, in v2, I will start with just two bits, NBD_INIT_SPARSE (entire image is sparse, nothing is allocated) and NBD_INIT_ZERO (entire image reads as zero), and save any future bits for later additions. Do we think that 16 bits is sufficient for the amount of initial information likely to be exposed? Are we in agreement that my addition of an NBD_INFO_ response to NBD_OPT_GO is the best way to expose initial state bits?
-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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