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SnapshotCM Newsletter for October, 2012


  • Editor's Blog
    • Copy or Merge?
    • New Release
  • Moving a Workspace
  • Recommended Releases
  • Links We Like

Editor's Blog

Copy or Merge?

What do the menu actions "Copy Changes...," "Copy Changes from Parent," "Promote Changes to Parent," and "Copy Changes To..." have in common? They are all used to merge changes unique to the source snapshot into the target snapshot. If you've used these actions in SnapshotCM, then you have experienced just how simply merge is accomplished in SnapshotCM, even if we call it something else.

However, the term copy doesn't fully express what happens during these actions. So we will be transitioning from the term copy to the term merge. Nothing new is happening, but we feel the change will more accurately communicate the action that is performed. To ease this transition, we've started by updating the terms used in the Compare Browser, but, for now, left the menu terms alone. Just remember, that Copy Changes == Merge Changes.

New release

We've also just posted an update to the 1.92 release which fixes a few reported defects and adds ignore filters to the workspace browser. We will discuss filtering more in a coming newsletter.

For more info on the new release, see the Change List

As always, we welcome your feedback and ideas.

Scott Kramer

Moving A Workspace

Recently, a customer reported that SnapshotCM "hung", after migrating their workspaces to a solid state drive. Investigation showed that the file system clone failed to copy the SnapshotCM's workspace cache files. The cache files, one per snapshot mount, store workspace state that allows SnapshotCM to efficiently interact with the workspace. When this information is missing, SnapshotCM automatically recreates it, which can be time-consuming as it involves checking out and comparing each file in the workspace with the corresponding file in the snapshot. Especially for very large workspaces, this can appear as a hang. If you run into this, the easiest solution might be simply to be patient. But if you have several large workspaces, read on for alternatives.

SnapshotCM cache files are stored, one per mount point, as an alternate data stream (ADS) on each mount directory when operating on a Windows NTFS file system. (On non-NTFS file systems, they are stored in the file .cache.SnapshotCM in each mount directory.) Since ADS files are normally invisible, it's easy to understand why copy or clone software might fail to copy them. In fact, Windows makes it pretty hard to see them. This is good in that it greatly reduces the likelihood that the cache files will be inadvertently removed or corrupted. And they are not "in the way." But when you want them, it takes special effort to see or copy them.

ADS files are not visible from Windows Explorer. You need to use the command-line (or a third-party utility) to see them. So fire up a CMD window and run dir /R to see them. The /R option lists ADS files, which show up with file names containing three, colon separated components. For the SnapshotCM cache, they look like mountdir:SnapshotCM:$DATA where mountdir is the name of a mount point directory in the workspace. Note if mountdir is a just one character, it must be prefixed with another directory to distinguish it from a drive specification (.\M:SnapshotCM works). The rest of the name identifies this as a data stream named SnapshotCM.

To view the contents of a SnapshotCM cache file, the easiest option is to run wdump -c [dir] where dir is any directory in the mount whose cache is to be displayed. wdump will then write the cache file to stdout. Alternatively, the more command can display the contents of an ADS stream. In our example, enter more < mountdir:SnapshotCM. Most Windows commands do not understand ADS files or the illustrated ADS syntax.

Windows tools, such as Windows Explorer and xcopy, correctly copy ADS data, even though Windows Explorer can't display ADS items. The target file system also must be NTFS to preserve the ADS. Disk cloning tools, including backup and restore solutions, may not copy/backup/restore ADS items correctly, leading to the loss of the SnapshotCM cache. If the ADS is not copied, or not supported in the target file system, or you are copying from a non-NTFS system to an NTFS system, then use the more command to copy the cache files from the old workspace to the new. For example: more < oldmountdir:SnapshotCM > newmountdir\.cache.SnapshotCM. Any such copying should occur before invoking any SnapshotCM tool on the copy. And once the cache is initialized, we strongly recommend that you leave it alone.

Bottom-Line Summary

For Windows NTFS to Windows NTFS copying or moving of workspaces: Use Windows Explorer or xcopy.

For Windows NTFS to non-NTFS: Before removing the original location, extract the ADS cache files into regular files. Once the copy/move is completed and before using SnapshotCM, copy the saved cache files into .cache.SnapshotCM in the corresponding mount directory.

For Non-NTFS to Windows NTFS: Copy or move the data, then before using SnapshotCM, copy the .cache.SnapshotCM files into the ADS stream on the mount directory.

For non-NTFS to non-NTFS (all non-Windows platforms): Copy or move the hierarchy. The .cache.SnapshotCM files should be moved or copied appropriately as part of the hierarchy.

For further study, we suggest Alternate Data Streams: Out of the Shadows and into the Light.

Recommended Releases

The following releases are recommended:

  • - The product-line release, with the latest fixes and performance enhancements.
  • - The last pre-product-line release.
  • 1.82.06 / 1.82.07 / 1.82.08 - stable version with the old (single mount) workspace model.

If you are running any other release, we recommend that you update to the latest recommended version that your license allows.

For a complete list of user-visible changes, see the Change List.

Links We Like

Links we find interesting, fun, or occasionally useful.

We are looking for interesting links to share. Send to

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