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Changes to backup source during backups

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Today a file was replaced during a full "Files and folders" backup.  I was sort of surprised to see that the changed file made it into in the backup even though the backup was nearly complete what the change happened.  So that got me wondering.  How does ATI handle changes to the source during a backup?  Does it check to see if changes were made during the backup, or was I just lucky that the backup had not yet gotten to the replaced file?

The source files and folders in question are on a NAS.  Does that make a difference?  I ask because I see "I0017001A: Error 0x17001a: Failed to create snapshot for backup." in every one of these (daily) backups.  I didn't know a snapshot was attempted for Files and folders backup, but obviously ATI tried. 

I don't see that error when taking a Files and folders backup of a local directory.  Does that imply that a snapshot was used for that backup?  (If so, a change to a file during the backup would not be caught.)

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Patrick, it should be using a snapshot which is meant to lock the drives state at the moment the snapshot is created and the backup started.

Are you using the vss method or the older Snapapi method?

It really shouldn't matter, but curious. VSS is a Microsoft snapshot method, so would be more surprised to see this happen here than with the older Snapapi method.

I haven't specified a snapshot option so I assume I'm using VSS (VSS without writers I assume since that article says that's the default for file-level backups).  However, I seem to recall that VSS is not supported for NAS drives.  I'll double check that, but maybe I should try Snapapi in the mean time.

Update:
I changed the backup to use the Acronis Snapshot but got the same error:

5/13/2019 1:26:07 PM: -07:00 7904 I00000000: -----
5/13/2019 1:26:07 PM: -07:00 7904 I00000000: ATI Demon started. Version: 23.5.1.17750.
5/13/2019 1:26:07 PM: -07:00 7904 I00640000: Backup reserve copy attributes: format tib; need_reserve_backup_copy false;
5/13/2019 1:26:07 PM: -07:00 7904 I00640002: Operation Networked Files started manually.
5/13/2019 1:26:09 PM: -07:00 7904 I00640000: Backup reserve copy attributes: format tib; need_reserve_backup_copy false;
5/13/2019 1:26:09 PM: -07:00 7904 I013C0000: Operation: Backup
5/13/2019 1:26:09 PM: -07:00 7904 I0064000B: Priority changed to Low.
5/13/2019 1:26:09 PM: -07:00 7904 I00010400: Child process has exited with code '0'.
5/13/2019 1:26:09 PM: -07:00 7904 I00640086: Execution of user command succeeded: //WDMyCloud/Public/Validation/Integrity_check.bat
5/13/2019 1:26:10 PM: -07:00 7904 I0017001A: Error 0x17001a: Failed to create snapshot for backup.
| trace level: information
| line: 0x616ad0806a6302a7
| file: c:\bs_hudson\workspace\790\archive\sel_dir.cpp:1436
| function: Seldir::SelectedDir::MakeSnapshot
| line: 0x616ad0806a6302a7, c:\bs_hudson\workspace\790\archive\sel_dir.cpp:1436, Seldir::SelectedDir::MakeSnapshot
| $module: ti_demon_vs_17750
5/13/2019 1:26:10 PM: -07:00 7904 I000B03F0: Create Backup Archive From: \\WDMyCloud\Public\
To file: Networked Files_inc_b23_s2_v1.tib Compression: Normal
5/13/2019 1:28:44 PM: -07:00 7904 I00640000: Writing incremental version to file: Networked Files_inc_b23_s2_v1.tib
5/13/2019 1:28:45 PM: -07:00 7904 I00640000: The following backups have been successfully created: F:\My Backups\Networked Files\Networked Files_inc_b23_s2_v1.tib
5/13/2019 1:28:45 PM: -07:00 7904 I013C0006: Operation has succeeded.

Start: 5/13/2019 1:26:07 PM
Stop: 5/13/2019 1:28:45 PM
Total Time: 00:02:38

I've been doing this backup for years without any noticeable problem so I'm not particularly worried.  Logs going back to Feb 2017 show the error.  (I'm not sure why I have no logs from before that.  Maybe I did a clean install of ATI then.)

I know nothing about the Acronis snapshot.  Should it work on a NAS?  I would think the NAS itself would have to take the snapshot.

Ah, OK, I missed that the source is also on the NAS.  Yeah, I don’t think a snapshot is possible on a remote device that has it’s own OS managing the permissions and access directly.  

What I’m reading online is that VSS is not supported over SMB to remote, non-Windows devices.  On some of these NAS devices, if you connect via ISCSI, then, VSS should be fine, assuming the LUN is mounted to the host system OS.

So, apparently, in this case, it’s using SBM (most likely) and as a result, it’s reverting to a backup that is basically just looking for any changes to the source, regardless if they occurred after the backup already started, or not.