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Subject: RE: [virtio-dev] Clarifications for the new DISCARD and WRITE ZEROES commands
Hi Jakub, The latest patch set for adding DISCARD and WRITE ZEROES to virtio-blk locates here, https://lists.linuxfoundation.org/pipermail/virtualization/2018-June/038299.html > -----Original Message----- > From: virtio-dev@lists.oasis-open.org [mailto:virtio-dev@lists.oasis-open.org] On > Behalf Of Jakub Jermar > Sent: Thursday, August 16, 2018 5:33 AM > To: virtio-dev@lists.oasis-open.org > Subject: Re: [virtio-dev] Clarifications for the new DISCARD and WRITE ZEROES > commands > > On 08/15/2018 05:47 PM, Paolo Bonzini wrote: > > On 15/08/2018 17:12, Jakub Jermar wrote: > >> In the absence of such a conforming patch, I would kindly ask someone to > >> clarify the relationship between the virtio_blk_discard_write_zeroes > >> structures and the virtio descriptor. [1] seems to indicate that there > >> is always one struct virtio_blk_discard_write_zeroes per descriptor, but > >> the wording is not very clear. If the containing descriptor must be a > >> multiple of 512 in size (because of u8 data[][512]), > > > > The descriptors are intended to be a multiple of 16 bytes in size for > > discard and write zeroes. > > > > In fact, even for reads/writes the descriptors are a multiple of the > > logical block size, not of 512. That ought to be corrected as well. > > > >> there is no way to > >> figure out how many of these structs it contains. So there must be only > >> one and the only way to add a segment is to chain descriptors. > > > > No, it does not matter how descriptors are chained. Data is always > > treated as if it was just one descriptor. > > > >> Unless > >> the descriptor size is somehow allowed to be a multiple of sizeof(struct > >> virtio_blk_discard_write_zeroes) in which case it could be determined > >> and there may be several of these structures per descriptor. > >> > >> Also, why is it that there is discard_sector_alignment but no > >> write_zeroes_sector_alignment? > > > > You can use discard_zeroes_alignment too if unmap=1 in the descriptor's > > flags. If unmap=0, it doesn't matter. > > Thank you, that was quite helpful. > > Jakub > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
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