On 09/02/2015 03:57 PM, Clint Olsen wrote: > I'm also interested in generating images to preserve the entire disk. > What I'm wondering is whether doing this on a live system through > Cygwin would produce a safe, bootable disk image or if the the APIs > that Cygwin has to use or having the disk mounted would make this > unreliable?
The answer is the same on non-cygwin systems. ANY attempt to sequentially read a disk while some other entity may be actively modifying the same disk is doomed to capture incomplete (and probably unusable) state. > > A friend speculated that dd might complain about open files or > some-such thing. I went and tried it on /dev/sda and it seemed to work > without complaining. dd won't complain, but it also won't capture an accurate image. There is a difference between the block layer (read each sector of the disk, regardless of what is happening in other sectors due to file system activity) and the file system layer (read multiple files one at a time, regardless of which sectors the file system currently maps those files to). The only safe way to use dd to clone a disk is if the disk is not currently mounted by any filesystem that might be actively modifying sectors in the block device. There are various solutions that CAN capture an accurate point-in-time disk snapshot. For example, on Linux, you can use LVM and create an LVM snapshot, which separates all data at a given point in time from all further changes to the live disk, so that you can then do a background task that reads the snapshot without worrying about data going inconsistent, and so that the guest can continue to run with the live overlay. Many SAN storage vendors sell specific solutions along these lines, and it is also a hot topic with virtual machine solutions. But in every case, these sorts of solutions require additional storage hierarchies (they are not copying /dev/sda directly, so much as exposing /dev/sda as a layer on top of other storage such that the actual snapshot operation utilizes that other storage for the background copy-on-write overlay magic). -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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