Re: [Libguestfs] Point-in-time snapshots

On 03/27/2015 09:35 AM, Richard W.M. Jones wrote: > But libguestfs doesn't want to do a backup, nor get a copy of the > whole disk, it just wants to access a scattering of blocks (maybe a > few hundred) but at a single point in time, in as lightweight a manner > as possible. If you KNOW what sectors you want to read, then your NBD target can ignore writes to the sectors you don't care about (the guest is changing data on a sector you don't care about; yeah, bandwidth was spent in telling you that, but you don't have to spend storage on it), while focusing on reading the sectors you do care about as fast as possible (to minimize the time spent dealing with uninteresting writes). If you DON'T know what sectors you want to read (because you are chasing file system pointers and don't know a priori where those pointers will resolve), then tracking ALL data flushed by guest writes IS the most efficient manner for keeping your point in time accurate for the duration of your fleecing operation. Either way, if you really are going to read only a few hundred sectors and then close the connection, it shouldn't matter if the drive-backup failed to send all guest sectors modified after the point in time, so long as every sector you read is accurate (either because the guest hasn't touched it in the meantime; or because even though the guest touched it after the point in time, you were given the original contents through your NBD target).