Re: [CentOS-virt] Remove Centos from AWS marketplace
On Sun, Mar 09, 2014 at 11:28:07AM -0400, Digimer 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. Database disk snapshots may include transactions in flight and the on-disk image may not be in a consistent state. Databases such as Oracle try to work around this by ensuring that writes occur in a specific order and have a good recovery process (each data file has a change number; determine the best change number to start from, roll forward from there to recover, then roll back any incomplete transactions) but it's considered crash recovery and shouldn't be part of BAU activity. Other databases may not be so good at recovery (mysql?) and so you run the risk of database corruption if you need to restore the snapshot. If you rely on disk snapshots then it's recommended you do a proper db dump before the snapshot is taken, so that you can recover the database from the dump file and not the snapshot. -- rgds Stephen ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Remove Centos from AWS marketplace
On Sun, Mar 9, 2014 at 11:28 AM, Digimer li...@alteeve.ca 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@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Remove Centos from AWS marketplace
On 09/03/14 11:52 AM, Nico Kadel-Garcia wrote: On Sun, Mar 9, 2014 at 11:28 AM, Digimer li...@alteeve.ca 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. This is very useful, thank you kindly for sharing. I suppose I always considered the it's like recovering for the server losing power as usually works and equating that to good enough backup. So I suppose, at best, using snapshot images as a backup ... backup method would be valid... I could see the benefit of recovering the VM, and then if anything wasn't right, using it as the target for restoring data from the proper backup. Thanks again! -- Digimer Papers and Projects: https://alteeve.ca/w/ What if the cure for cancer is trapped in the mind of a person without access to education? ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
Re: [CentOS-virt] Remove Centos from AWS marketplace
On 09/03/14 11:43 AM, Stephen Harris wrote: On Sun, Mar 09, 2014 at 11:28:07AM -0400, Digimer 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. Database disk snapshots may include transactions in flight and the on-disk image may not be in a consistent state. Databases such as Oracle try to work around this by ensuring that writes occur in a specific order and have a good recovery process (each data file has a change number; determine the best change number to start from, roll forward from there to recover, then roll back any incomplete transactions) but it's considered crash recovery and shouldn't be part of BAU activity. Other databases may not be so good at recovery (mysql?) and so you run the risk of database corruption if you need to restore the snapshot. If you rely on disk snapshots then it's recommended you do a proper db dump before the snapshot is taken, so that you can recover the database from the dump file and not the snapshot. Thanks for the reply, Stephen. I also replied to Nico, and my comments there can be directed to you, as well. :) -- Digimer Papers and Projects: https://alteeve.ca/w/ What if the cure for cancer is trapped in the mind of a person without access to education? ___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
[CentOS-virt] Remove Centos from AWS marketplace
https://forums.aws.amazon.com/thread.jspa?messageID=481859#481859 https://forums.aws.amazon.com/thread.jspa?messageID=453572#453572 This is a timebomb waiting to strike so many people who like do daily snapshot backups and keep them for few weeks and not realizing their snapshots are useless if they had accidentally mess up some boot related file earlier on. Another scenario you mess up the sudoers file or the root authorized keys - you'd have to loose a whole days data and go to previous nights restore just for single file error like that ? If AWS marketplace is unable to remove this hardcoded rule then it's only prudent to remove Centos from AWS marketplace and release it in the community section instead ? Thoughts ?___ CentOS-virt mailing list CentOS-virt@centos.org http://lists.centos.org/mailman/listinfo/centos-virt
