Why no Linux version?


Author
Message
Mintmag
Mintmag
New Member
New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)
Group: Forum Members
Posts: 28, Visits: 122
Just curious is all. I keep hearing how Linux is so great just wandering why it's not support more by commercial IT software such as this? 
jphughan
jphughan
Most Valuable Professional
Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)
Group: Forum Members
Posts: 3.3K, Visits: 24K
Because Linux doesn't have an equivalent to VSS, which is the only way that image backups of a live system can safely be captured.  Reflect does however support capturing and restoring images of partitions that use Linux file systems, as well as cloning them, all of which you can do from the Rescue Media environment or even from Windows on the same PC if you have a dual boot setup.  You simply can't run Reflect in Linux, but the inability to image a live system clearly reduces the value proposition of committing engineering resources to developing a Linux-native version, since you'd have to boot into another environment to image that system anyway.  I don't believe F&F backups are supported with Linux partitions, but Linux already has plenty of file-level backup solutions.

Edited 26 June 2018 2:02 PM by jphughan
Mintmag
Mintmag
New Member
New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)
Group: Forum Members
Posts: 28, Visits: 122
I thought it had something to do with VSS. That's a Windows only feature right? The same thing used in System Restore?
Edited 27 June 2018 4:04 AM by Mintmag
jphughan
jphughan
Most Valuable Professional
Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)
Group: Forum Members
Posts: 3.3K, Visits: 24K
Yes, VSS was developed by Microsoft and is therefore only available in Windows -- but not only is there no VSS in Linux, there isn't a full equivalent to it either.  I'm not sure why that is.  System Restore does indeed rely on VSS snapshots, but the use cases for VSS snapshots extend far beyond that.  Windows Server versions (and the client version of Windows 7) use VSS snapshots to enable a Previous Versions / Shadow Copies function that allows you to retrieve previous versions of files that are maintained thanks to VSS snapshots.  And then VSS not only takes snapshots of volumes but coordinates with applications before doing so, it can enable reliable backups even of files (and in the case of Hyper-V hosts, entire VMs) that are open and running during the backup, without risk of capturing the data in an inconsistent and therefore corrupted state.  I wrote a PM to someone explaining in a medium level of detail how VSS works and is able to guarantee capture of data in a consistent state in scenarios like this if you're curious:

- An application like Reflect requests a VSS snapshot.
- Windows notifies all applications that have registered themselves as VSS-aware that a snapshot has been requested. They start queueing new writes intended for the disk to memory instead, and then their VSS writers do whatever they're programmed to do in order to get the data that's actually on disk into a consistent state. This might include committing certain data currently in memory to disk and sometimes even deleting data. Windows Search deletes its own index database in VSS snapshots, for example, because it can grow quite large and can be recreated automatically if it's lost anyway. Hyper-V either tells its running guest VMs to perform their own VSS snapshots so they get their own VMDKs into a consistent state, or for guests that don't support that, it briefly pauses the VM and captures an "application-consistent" snapshot, which includes the contents of the VM's memory space (which can take a while to create).  This is why you can run a backup of a Hyper-V host and guarantee consistent data backups even of running VMs.
- When the each writer has got its data tidied up on disk, it notifies Windows that it's ready for a snapshot to be taken.  When all registered VSS writers have confirmed they're ready for a snapshot, Windows takes the snapshot.  It then notifies VSS-aware applications that they can start writing to disk again rather than queueing in memory (and that Hyper-V can resume any paused VMs), and finally notifies the original requesting application (like Reflect) that the snapshot is ready for use.

In terms of how new data is written afterward, it doesn't get redirected to a delta file anywhere. Instead, for any write that will modify existing data, the current data on disk is copied to the snapshot area before the new write is committed -- hence the term "copy-on-write". As a result, a VSS snapshot isn't created as a fixed entity at the time of the original capture; it's actually a living entity that grows after it's established -- until the next VSS snapshot occurs, whenever that is. Because of this design, and the fact that the amount of space allocated to storing VSS snapshots is limited (by default it's 10% of the volume size), if you have an application like Reflect that's trying to read from the snapshot, and you also had a ton of write activity going on while the backup was running, it's possible that that single snapshot could exceed the entire allocated storage area, even after purging older snapshots. If that happens, Windows will delete the VSS snapshot, and the Reflect would fail with an error explaining that. But the advantage of this design is that you don't have to redirect writes to deltas anywhere and commit them later; you instead just archive the current data before you modify it. It's weird to think that Reflect can capture a consistent backup by working from a snapshot that's evolving as Reflect is reading from it, but that's how VSS works.

Edited 27 June 2018 4:14 AM by jphughan
Mintmag
Mintmag
New Member
New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)
Group: Forum Members
Posts: 28, Visits: 122

That was quite the lengthy read but thanks for all the details. What about programs that aren't VSS aware. They still seem to work and everything after restore. Though I usually close down every program before starting an image. 
jphughan
jphughan
Most Valuable Professional
Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)
Group: Forum Members
Posts: 3.3K, Visits: 24K
I'm not entirely sure what VSS does with applications that aren't VSS-aware.  Someone who knows more about it may weigh in here, but my guess is that if a non-VSS application is in the middle of a crucial write when the snapshot is captured, then the file it was writing would be corrupt or at least inconsistent.  Or in the case of a brand new file being written to disk, e.g. from a file copy in progress, the file might simply not exist in the snapshot at all.  Maybe Windows will wait a certain amount of time for a "quiet period" on disk where writes aren't occurring in order to capture the snapshot as safely as possible?

The two biggest application use cases for VSS are probably Exchange and SQL, where VSS is crucial because both can be writing new data constantly and capturing a partial write could compromise the integrity of a large database.  But the average PC application isn't constantly writing data that's critical and where a mid-write snapshot would do a lot of damage -- and if a particular application IS doing that, then hopefully its developer created a VSS writer for it.  Still, when feasible it's definitely not a bad idea to keep the number of running applications to a minimum at least while the snapshot is being created.

Edited 27 June 2018 5:16 AM by jphughan
Froggie
Froggie
Master
Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)Master (1.5K reputation)
Group: Forum Members
Posts: 844, Visits: 7.1K
JP is correct.  With all imager's early response to non-VSS environments and attempted consistency with file content, they all (even Reflect's early PSsnap.sys driver) tried to wait for a quiet period before locking their own snapshot areas for imaging purposes.  The only serious victims of those types of snapshotting were DataBases mostly, and database lookalikes (Outlook).  The result could be inconsistent DATA files causing some sort of havoc along the way.

VSS and VSS-aware applications keep that from happening by flushing all their databases prior to reporting back to VSS that they're ready for the snapshot.
Mintmag
Mintmag
New Member
New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)
Group: Forum Members
Posts: 28, Visits: 122
I have a bunch of games installed on my computer. They don't seem to be affected though.
jphughan
jphughan
Most Valuable Professional
Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)Most Valuable Professional (4.7K reputation)
Group: Forum Members
Posts: 3.3K, Visits: 24K
Why would you expect them to be?
Mintmag
Mintmag
New Member
New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)New Member (43 reputation)
Group: Forum Members
Posts: 28, Visits: 122
jphughan - 27 June 2018 10:45 PM
Why would you expect them to be?

I don't think they were made with VSS in mind. Or maybe I don't quite understand how VSS works with programs
GO

Merge Selected

Merge into selected topic...



Merge into merge target...



Merge into a specific topic ID...




Similar Topics

Reading This Topic

Login

Explore
Messages
Mentions
Search