On 3/30/20 10:27 PM, Matthew Ahrens via openzfs-developer wrote: > On Mon, Mar 30, 2020 at 7:08 PM Richard Laager <rlaa...@wiktel.com > <mailto:rlaa...@wiktel.com>> wrote: > > My only personal interest in O_DIRECT is for KVM qemu > virtualization. It > sounds like I will probably need to set direct=disabled. > Alternatively, > if I could get all the writes to be 4K-aligned (e.g. by making all the > virtual disks 4Kn?), then ZFS's O_DIRECT would work. > > > We were thinking that qemu *would* be able to use O_DIRECT, or at > least it wouldn't need direct=disabled. But I think your assessment > implies that qemu usually uses O_DIRECT i/o that is not page (4K) aligned
Yes, that was my assumption. Imagine the (likely still typical) case of 512B virtual disks. If the guest does a 512B write, is KVM really doing RMW to make that 4K? I'm assuming not. This very well may be a faulty assumption. I'm not a qemu developer. -- Richard ------------------------------------------ openzfs: openzfs-developer Permalink: https://openzfs.topicbox.com/groups/developer/T950b02acdf392290-M09e05d22672a2f0e1688853e Delivery options: https://openzfs.topicbox.com/groups/developer/subscription
