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Volume Shadow Copy For Linux?
An anonymous reader writes "I was asked to manage a number of Linux servers at work. I would like to use volume snapshots to improve my backup scripts and keep recent copies of data around for quick restore. I normally manage Windows servers and on those I would just use Microsoft's Volume Shadow Copy for this. I tried Linux LVM snapshots, but most of the servers I manage run regular partitions with ext3 file systems, so LVM snapshots will not work. I found some versioning file systems out there like ext3cow and Tux3. Those look interesting, but I need something I can use on my existing ext3 file systems. I also found the R1Soft Hot Copy command-line utility, but it does not yet support my older 2.4 Linux servers. What are you using to make snapshots on Linux?"
Linux servers don't get backed up, they only get migrated to new hardware every five years. (BSD is the same, except for the second part.)
If you have backups, then moving to LVM is obviously the way to go if you desire snapshots. The others options are short-term hackery, LVM was designed from the ground up to do such things. And Ext3 has nothing to do with the price of butter.
To clarify, let me rephrase your question for the other way around
"I was asked to manage a number of *Windows* servers at work. I would like to use volume snapshots to improve my backup scripts and keep recent copies of data around for quick restore. I tried Windows Shadow Copy, but most of the servers I manage run MBR partitions with FAT file systems, so Shadow Copy will not work. I found some versioning file systems out there... Those look interesting, but I need something I can use on my existing FAT file systems. I also found --random freeware--, but it does not yet support my older Windows NT 3.5 servers. What are you using to make snapshots on Windows?"
Except, in that case, it makes more sense because the filesystem is the determining factor, not the volume management. If you have LVM, it doesn't matter what the underlying filesystem is, really. Stop faffing about - if you have a server, with backups, that you need snapshots on, take the hit and wipe the drives to a config that supports that... while you're there upgrade that damn kernel already. If nothing else, it will test that the backups you're making are actually worth the effort. It's like complaining that 95 on FAT16 doesn't support Shadow Copy. If you absolutely *can't* take those servers down, or am unable to restore your backups to another machine for testing such changes (whether because of compatibility, software licensing and/or bad backups), you have bigger problems than some random desire for a feature you don't actually *need* at the moment.
If they are indeed regular partitions, he can't use LVM snapshots. However, the best solution is to convert from partitions to LVM volumes. It's a little effort to do so, but switching is worth it. Second best is to wait until btrfs is more usable. As a ZFS user, I can say that filesystem-level snapshot are much nicer than LVM snapshots in lots of ways.
Another possibility is to abuse hardlinks. You can create a copy of a directory with cp -al, and then overwrite (not modify) files on the original, you'd have copy-on-write copy. If you make your backups with rsync, you can configure rsync to never write to existing files and always overwrite, then use cp -al each weak or day, to store "incremental" backups for weeks and maybe more. I personally found this solution nice, but then I installed Solaris on the backup machine and used ZFS snapshots which do the same safer, simpler and more efficiently. If the backups are stored on a separate machine, switching it to Solaris is an option.
Another thing that can be done is to keep the LVM snapshots on a separate machine, and leave the current partitions as they are. It can be done with rsync, or a drdb device can be used to sync with the server (it can be created without reformatting the partitions, but you still need to make some changes like shrinking and/or moving the data, which might destroy the data if you don't know what you're doing).
He isn't complaining. You seem to be responding to his mentioning that "he knows how to do this on Windows" , by interpreting it as "Why is Linux so broken that it can't do a simple thing like that?" This isn't a Linux versus Windows thing. This is a Windows user, migrating to Linux and wants to know how to accomplish something. Constructive answers are more useful in such cases than getting defensive by alleging hypocrisy and double standards.
Entia non sunt multiplicanda praeter necessitatem.
Windows does it in precisely the way uncoordinated open source projects will never be able to do it. They told the NTFS team, a new team in charge of the volume snapshot service, and the team in charge of the logical disk management to work together, create and perform regression tests against each others code on every check-in and patch, and likely set up team liaisons whose sole purpose was to ensure interoperability.
If you told me a third party open source organization, without having full control of the developers and direction of both the ext filesystems and the LVM system, was going to write a service that performed the same function as the volume snapshot service, I would laugh at you. I would laugh and laugh. Open source, because of its nature, tends to attract developers who want to do something, and they want it to be the best at that something. At the same time, they don't want to tie themselves down to stable APIs because, well, it can be limiting and slow development. I totally understand why. So telling me that some third party is going to extend LVM with one API, EXT with another API, and then write a service to coordinate the two is mind-boggling. Those people would have to constantly commit code to match changes in either of the two rapidly changing projects, they'd have to fully understand the inner workings of both ext and LVM, and then they'd have to make it all work without corrupting anyone's data and ruining their reputation beyond repair.
On the other hand, you have projects like ZFS or BTRFS, which are just as monolithic, but more ambitious, and powered by the same developers I mentioned above. They want their solution to be the best. It takes a long time though because they essentially have to start from scratch and incorporate all the things that appear to be within arm's reach. But the people who start projects like BTRFS realize that it's a fool's errand to try and create interoperability between massive, disparately managed open source projects. GNOME and KDE only survive because they threw everything else out and decided to simply come with their own full suite of stuff. X is its own long story.
I don't want to diss open source, like I said, it creates magnificent pieces of software, and the developers really, truly tend to care about their projects. (Even if they can be a little defensive, sometimes.) But without a dictator forcing cooperation between different teams, you often see open source reinventing the wheel. Sure, LVM and EXT3 could theoretically work together to provide sane, fast, performant snapshots. But I'd like to meet the person who thinks they can pull that project off.