On 10/29/2012 09:54 AM, Kashyap Chamarthy wrote: > On 10/28/2012 02:07 AM, Jorge Fábregas wrote: >> Hi there, >> >> I'm just curious: Where does the "Copy on Write" (from QCOW) comes >> from? Everything I've been reading regarding how snapshots work with >> QEMU points to a "redirect on write" approach. Or is the COW part >> related to some other internal mechanism (not related to snapshots)? > > The COW part /is/ related to external disk snapshots:
Now, that I re-read your question (after taking coffee), I didn't answer your question :) I think, COW means -- on 'write', modify the 'copy'(which is the overlay). /kashyap > > Whenever you take external disk-only snapshot, the current disk image in use > is marked > read-only(while serving as a backing file for the rest of the snapshots > taken), and a new > QCOW2 overlay is created to track the new 'writes'(or delta) from there on. > > It's worth noting that, the base image can be RAW or QCOW2, but the > snapshots(or overlays) > are always QCOW2. > > (Note that - read-only doesn't mean, file-system permissions, but 'qemu' > opens the disk > read-only.) > > > See if this are helpful : > > http://kashyapc.wordpress.com/2012/09/14/externaland-live-snapshots-with-libvirt/ > > http://kashyapc.wordpress.com/2012/10/04/creating-rapid-thin-provisioned-guests-using-qemu-backing-files/ > > /kashyap > >> >> Thanks in advance, >> Jorge >> _______________________________________________ >> virt mailing list >> [email protected] >> https://admin.fedoraproject.org/mailman/listinfo/virt >> > > _______________________________________________ > virt mailing list > [email protected] > https://admin.fedoraproject.org/mailman/listinfo/virt > _______________________________________________ virt mailing list [email protected] https://admin.fedoraproject.org/mailman/listinfo/virt
