On Sun, Mar 9, 2014 at 11:28 AM, Digimer <lists@xxxxxxxxxx> wrote: > Would you mind elaborating on this? If a snapshot is a point-in-time > image of a VM (or even normal FS), why would DB backups be at risk > (assuming things like fsync are used)? > > I'm asking in general terms... no idea if this is something AWS specific. > > digimer It's a general issue. If a system snapshot is used to correctly preserve both the disk image, and the "state" of the VM including memory, well and good. The state is recoverable. There's always a risk that interrupted network transactions left things in an unexpectedly inconsistent state that the VM is not equipped to handle: I'm thinking particularly of "wget" or other download transactions where the download software was not intelligent enough to verify the download before proceeding. I've been through this a lot lately with "chef" software. It's compounded by network based filesystem transactions, such as interactions with NFS or CIFS filesystems, which cannot be synchronized with the OS snapshot. But simply relying on the disk image from such an AWS snapshot, without recovering the full system state, is a potential adventure. I've not myself had opportunity to play with this kind of restoration, so I'm uncertain whether AWS allows access to the plain disk image, or automatically would bring the full VM state with it for re-activation of the snapshot. If you're just getting at the disk images, using "fsync" before the snapshots is helpful, but any atomic transaction that is in progress at the time of the disk image snapshot is not verifiable in the atomicity of that transaction. This particularly includes precisely the sort of "page mapped" data, sitting in RAM, that the "fsync" command helps write to disk. And snapshots cheduled from outside controllers, such as automatic snapshots, cannot be reliably synced with system specific "fsync" database suspension commands without a great deal of integration between the outside system, and the local host, that VM's are not supposed to normally need. I went through great deal of this some years back, shutting down databases, running "LVM" to get a disk snapshot, then running "rsnapshot" against the *snapshot* to avoid getting an inconsistent state of the database into the backup system. And there are some *funky* databases out there. Ask sometime about the "Use hardlinked RCS files for source control of multiple project branches" sometime, if you'd like to wince a lot. _______________________________________________ CentOS-virt mailing list CentOS-virt@xxxxxxxxxx http://lists.centos.org/mailman/listinfo/centos-virt
