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Re: [Qemu-block] [Qemu-devel] [PATCH RFC 1/2] virtio-blk: add DISCARD an
Re: [Qemu-block] [Qemu-devel] [PATCH RFC 1/2] virtio-blk: add DISCARD and WRITE ZEROES features
Sun, 27 Jan 2019 12:51:44 +0000
On Fri, Jan 25, 2019 at 05:18:13PM +0100, Stefano Garzarella wrote:
> On Fri, Jan 25, 2019 at 02:58:56PM +0000, Stefan Hajnoczi wrote:
> > On Thu, Jan 24, 2019 at 06:23:22PM +0100, Stefano Garzarella wrote:
> > > + virtio_error(vdev, "virtio-blk discard/wzeroes header too
> > > short");
> > > + return -1;
> > > + }
> > > +
> > > + sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &dwz_hdr.sector);
> > > + bytes = virtio_ldl_p(VIRTIO_DEVICE(req->dev),
> > > + &dwz_hdr.num_sectors) << BDRV_SECTOR_BITS;
> > Please handle num_sectors << BDRV_SECTOR_BITS overflow so that we don't
> > need to worry about what happens with an overflowed value later on.
> Should I also check if num_sectors is less than "max_discard_sectors" or
Yes, anything that virtio_blk_sect_range_ok() doesn't check needs to be
checked manually in this patch. I can't see a reason for allowing
requests through that exceed the limit.
> Do you think make sense flush the MultiReqBuffer before call
> blk_aio_pdiscard() or blk_aio_pwrite_zeroes()?
I don't think that is necessary since virtio-blk doesn't guarantee
ordering (the legacy barrier feature isn't supported by QEMU).
Even the MultiReqBuffer is flushed, -drive aio=threads (the default
setting) could end up submitting discard/write zeroes before "earlier"
requests - it depends on host kernel thread scheduler decisions.
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[Qemu-block] [PATCH RFC 1/2] virtio-blk: add DISCARD and WRITE ZEROES features, Stefano Garzarella, 2019/01/24