TweetFollow Us on Twitter

Do You Copy?

Volume Number: 22 (2006)
Issue Number: 8
Column Tag: Field Notes

Do You Copy?

Are you copying all the information about your files that you need?

by Dan Shoop

Introduction

The Macintosh environment has always been feature rich, and that extends to its files, file attributes, and file metadata the file system maintains for them. OS X has introduced Macintosh to a wider array of application environments which, aside from that of Classic, include beasts such as Carbon, Cocoa, BSD, X-Windows, Web Objects, Java, QuickTime, and now Rosetta. Additionally, OS X supports different file systems that support different metadata. All of these may see and consider files differently than others. This results in files being copied rather differently.

This article is a short attempt to attune application developers, scripters, systems programmers, and file system architects to considerations relating to file metadata stored by the file systems on Mac OS X.

Philosophic Questions

In many respects it is a philosophic question as to what each environment on the Mac should properly copy. Some environments may not normally or naturally have access to all file metadata that OS X maintains for any given file. Named Extended Attributes may not be accessible through Carbon, Cocoa and Core Foundations, while they may be accessible from BSD. The nature of the operation may color and determine what metadata is important and what makes sense for preserving as a copy or whether the metadata should be reflective of a whole new file.

Consider the subtle differences of what metadata, or even files, get maintained as part of the different functions of operations like copies vs archives vs backups vs clones vs sync vs snapshots, et cetera. Is a copied file a new file or is the copy merely a replica or clone? This isn't as easy of a question as it first seems. For instance, if you copy a file from one location to another, does it make sense to maintain any ACLs it may have had or shouldn't the new file have its own, new ACL? But now, what if that "copy" was a backup or clone and part of a larger set of files in its directory? Wouldn't you then expect the ACLs to be maintained? Likewise, if you're backing up a set of files would you expect metadata associated with the directory (like sort order, icon positions, etc) to be maintained for the files? Yet, these are stored in places like .DS_Store or the Desktop Database, not as metadata for the file itself.

What about when we copy files to non-HFS+ file systems? What should be maintained? What should be expected if a file is copied off, then gets copied back? The Macintosh has defined the Apple Double format to store a file non-natively. Is it right for a Windows sysadmin to delete ._file's or .DS_Store's found on their file systems; is this "clutter"? To the Mac user who's lost the CODE resource of an application file, icons or font resources, a file's creation date, or creator/type you may end up with torches and pitchforks at your door if you're inconsiderate, and confusing if your tools are inconsistent. But should an ACL get copied to a foreign file system? Maybe not, but other Extended Attributes clearly are in Apple Doubles.

Moreover, consider what file creation, copying, and modification may mean. For instance consider that cat > file is often seen as creating a new file, but that if 'file' already existed then this operation is in fact a "modification".

Lastly, in the future we may be limited as to what we may and may not copy through DRM. This may become a technical rather than semantic issue as "secure" file systems and files are deployed (as in secure for the owners of the content).

Mac OS X File Metadata and Attributes

Briefly, let's run down some of the most significant file metadata and attributes common to Mac OS X.

Data Streams

The data fork is well known, but all Macintosh files have at least two forks, a data fork and a resource fork, either of which may be of zero length. Implied is that both exist. Under HFS+ multiple named streams may exist, which may be named, but the data and resource streams may not be renamed. Until Tiger, this was merely semantics.

Traditional views of files are just a single byte stream of named data (like found on a tape) with beginning-of-file (BOFs) and end-of-file (EOFs), and little to no metadata. The metadata was just "accounting" information used by the file system, it wasn't really "the data". However, over the years additional information was tacked on by file systems. Users got used to this additional metadata for cataloging and sysadmins got used to preserving them on backups. Environments became more complex and file systems started offering better ways to store ancillary data. The Macintosh isn't unique in offering multiple streams per file. Even Windows' NTFS has such provisions. HFS+ defined an architecture for "unlimited" numbers of streams or forks using the Attributes B-tree structure on the volume.

Under OS X the mandatory data and resource forks of a Macintosh file are exposed on HFS+ volumes as filename and filename/..namedfork/rsrc to BSD environments and as filename and ._filename when "split" (as for use on foreign file systems.) It also introduced tools for splitting and recombining forks split into what are arguably Apple Doubles.

Mac OS X 10.4 Tiger introduced the concept of Extended Attributes which implemented and exposed the additional named forks or streams on HFS+ volumes and through the 'split' fork ._filename on foreign file systems. ACLs are in turn implemented as a privately named and protected Extended Attribute and hence are a third fork that files may have on file systems that have ACLs enabled. Additionally any number of additional "Extended Attributes" (forks) may now be added to or read from a file with setxattr(), getxattr(), listxattr(), and removexattr().

Finder Flags

See `man GetFileInfo` for details, but these include file creator and type, attributes bits (such as locked, stationary, invisible), creation date and modification date. Stored in the ._filename Apple Double half on file systems other than HFS and its derivatives.

POSIX

Files have ownerships (user and group) and permissions. Symbolic links also have these attributes. Unix files also traditionally carry dates such as atime, mtime, and ctime, namely, the time of last access, last time file modified, and change time (last time the file's inode was changed), respectively. Note that ctime changes after operations like chmod.

Creation Dates

The subject of creation dates on the Mac warrants detailed discussion in itself. The Mac has always maintained file "creation date" metadata, it's a Finder file attribute, copied along with the file by the finder, displayed in the Finder's Get Info dialog and important enough that it is supposed to be included as part of the Apple Double file information stored in a ._file. But, while Mac users are long familiar with it and often use it to categorize files it's a rather foreign concept to unix (mostly because it traditionally never existed there), and some unix pundits believe it's just woolly-thinking (see http://mail-index.netbsd.org/netbsd-users/2000/11/22/0000.html) or at best the product of the confused (see http://toadstool.se/journal/2006/01/11/the-fallacy-of-ctime). Other unices and file systems support a btime (birth time) with varying degrees of success.

However, Mac users are used to creation dates, and may get rather agitated if they're lost or, say replaced with the modification time (which seems to be the OS X BSD environment behavior). As such, applications should strongly consider properly handling this metadata or, if they do mangle it, do so in a manner that lends itself to notice (like beginning of epoch).

For a more detailed commentary of Mac file creation dates see http://blog.plasticsfuture.org/2006/06/27/mac-os-file-creation-dates/

BSD Flags

BSD has traditionally maintained a series of flags associated with properties of a file as manipulated by `chflags`. These include flags such as the 'system immutable flag' (schg) that prohibit a file from being modified, or the 'archive flag' (arch).

The classical Desktop Database and OS X .DS_Store files

Traditionally the Macintosh stored certain metadata regarding files in the Desktop Database. Under Mac OS X the .DS_Store file associated with a directory contains similar information such as directory background info, the position or order of files and their icons within a directory, and more. When copying a directory you should probably consider copying these.

Lastly there's one Finder-level piece of information stored, disassociated from the file, which brings us to...

Spotlight Metadata

Tiger introduced us to Spotlight and a huge amount of application specific metadata stored that is associated with a file based on its content. This data is indexed by the kernel through 'importers' as a part of normal operations on the file.

While most of this doesn't need to be preserved since it is indexed as needed by the kernel, one attribute exposed to the Finder and stored in .DS_Store is what traditionally was known as the file Comments, called under Tiger the Spotlight Comments, and is displayed through the Finder's Get Info dialog.

How to preserve file metadata

There a few primary methods by which file metadata get preserved:

  • copied natively
  • preserved in special containers or lists
  • preserved by images of the volumes that support them (e.g. .dmg's)
  • split as Apple Doubles

BSD copies files using copyfile() which splits to Apple Doubles as necessary either internally or externally (with varying results)

Note that, currently, copyfile() munges the modified date into the creation date field of its Apple Doubles.

What needs to be copied? What doesn't?

While mostly a philosophic issue, it's probably safe to say that traditional Macintosh attributes such as creator and type are becoming less important since OS X has various mechanisms for associating files with their applications (sometimes at the cost of additional metadata) while other metadata, like creation date, should arguably be maintained. Spotlight metadata looks like a good candidate for general exclusion since the kernel will automagically recreate this for installed application importers on the target machine, but all Mac OS X systems might not have the same importers installed. This may or may not matter. Old Desktop Database files probably aren't an issue either as Classic withers or is mooted by Intel Macs, but .DS_Stores should probably be maintained if you're backing up a volume. Mandatory file forks should always be copied, but while these are Extended Attributes you might consider not copying the other forks that may exist, such as ACLs, depending on if you're backing up or cloning, or, if you're just duplicating the file elsewhere on a volume. It's hard to tell currently if other Extended Attributes need copying since they're rarely seen in action.

What Copies What?

Let's take a look at what metadata, some common tools and their operations preserve. The following is not designed to be a "report card" and it's not my goal to rank or rate a tool for its handling of the data, but sysadmins should be aware of how operations they perform may affect their data.

Coverage of various third-party backup applications is beyond the scope of this article, but it seems they fare rather poorly with regard to not preserving Extended Attributes aside from resource forks. Creation dates are a mixed bag and may vary within the application based on operation (sometimes intentionally). For additional discussion, see http://blog.plasticsfutures.org/2006/03/05/the-state-of-backup-and-cloning-tools-under-mac-os-x/

In general, copyfile(), which most all BSD tools rely on, does not perform consistently compared to the operation of the Finder. Creation date is clobbered by the modification date despite that copyfile() attempts to use Apple Doubles 'internally' and externally to foreign file systems and the Apple Double format specifically calls for the creation date. The various tools that use copyfile() then quite often fail to properly maintain internal or synthetic files resulting in lost Extended Attributes and other data. In rare cases the tool may crash, as in the instance of trying to `rsync -aE` a file with both ACLs and an additional non-resource fork, named Extended Attributes.

Apple System Restore (asr) in device mode manages to copy all the tested data, but this is to be expected since it is essentially a complete copy of the device. In file mode, however, asr, Disk Utility, and hdiutil have digressed in their behavior with OS X 10.4.6. Where previously it maintained locks (the Finder "L" attribute and BSD uchg flags), Extended Attributes and ACLs; it no longer preserves them, though this may change in a future update or (although this makes packages and dmgs problematic) it may be the intended behavior.

`dd` fares pretty much as you'd expect. At least philosophically it's understandable.

Editor's Note: To download a copy of the folloing table for reference please visit the MacTech source code ftp and select this month's issue, 22.08 :

http://www.mactech.com/editorial/filearchives.html



Figure 1. The changes in application code structure


Dan Shoop is principal researcher at iWiring (www.iwiring.net) and a Systems and Networks Architect for US Technical Services (ustsvs.com), both of which provide service and support for the Macintosh and other computer systems. He may be reached at <shoop@iwiring.net>.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

NeoFinder 7.0 - Catalog your external me...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Coda 2.6 - One-window Web development su...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more
File Juicer 4.51 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. In computer... Read more
PDFpenPro 8.3 - $124.95
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Create fillable forms and tables of content... Read more
PDFpen 8.3 - $74.95
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
BetterTouchTool 1.963 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
iFFmpeg 6.2.2 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
ForeverSave 2.1.6 - Universal auto-save...
ForeverSave auto-saves all documents you're working on while simultaneously doing backup versioning in the background. Lost data can be quickly restored at any time. Define your preferred time... Read more
NTFS 14.3.318 - $19.95
This latest version supports the new macOS 10.12 Sierra! NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and... Read more
BetterTouchTool 1.961 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more

Latest Forum Discussions

See All

Clash of Clans is getting its own animat...
Riding on its unending wave of fame and success, Clash of Clans is getting an animated web series based on its Clash-A-Rama animated shorts.As opposed to the current shorts' 60 second run time, the new and improved Clash-A-Rama will be comprised of... | Read more »
Leaks hint at Pokémon GO and Starbucks C...
Leaked images from a hub for Starbucks employees suggests that a big Pokémon GO event with the coffee giant could begin this very week. The images appeared on Reddit and hint at some exciting new things to come for Niantic's smash hit game. | Read more »
Silent Depth Submarine Simulation (Game...
Silent Depth Submarine Simulation 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: | Read more »
Enneas Saga lets you lead your own demon...
Defend the land of Enneas Continent from the forces of evil in the new fantasy MMORPG from Lyto Mobi: Enneas Saga. Can’t wait? No problem. It’s available to download now on Android devices. | Read more »
Great zombie games in the spirit of Dead...
Dead Rising 4 arrives tomorrow, giving enthusiasts a fresh chance to take selfies with zombies and get up to other ridiculous end-of-the-world shenanigans. To really get into the spirit of things, we've gone and gathered the best zombie games that... | Read more »
Amateur Surgeon 4 Guide: Advanced tips a...
Amateur Surgeon 4 is still tackling the competition at the top of the App Store charts, so if you haven't tried it out yet, you should probably do that right away. If you've been at it for a while, though, perhaps you're ready to start expanding... | Read more »
Amateur Surgeon 4 Guide: Become the worl...
It's time to wield your trusty pizza cutter again, as Amateur Surgeon has returned with a whole fresh set of challenges (and some old, familiar ones, too). Starting anew isn't easy, especially when all you have at your disposal is a lighter, the... | Read more »
Le Parker: Sous Chef Extraordinaire (Ga...
Le Parker: Sous Chef Extraordinaire 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Telltale Games really is working on a Gu...
Telltale Games' next episodic adventure is indeed Guardians of the Galaxy. A document tied to the voice actors strike suggested that the project was in the work, but now we have direct confirmation following an announcement at the Game Awards that... | Read more »
Amateur Surgeon returns to iOS and Andro...
Amateur Surgeon and its two sequels disappeared from the App Store some time and it was sad days for all. But now, just in time for the holidays, the Adult Swim favorite makes its joyous return in the shape of Amateur Surgeon 4, a remake with... | Read more »

Price Scanner via MacPrices.net

Holiday sale, Apple iMacs for up to $200 off...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more
Holiday sale: Mac minis for $50 to $100 off M...
B&H Photo has Mac minis on sale for up to $100 off MSRP free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $629 $70 off MSRP - 2.8GHz Mac mini: $899 $... Read more
Mac Pros on sale for up to $300 off MSRP, ref...
B&H Photo has Mac Pros on sale for up to $300 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3699, $... Read more
12-inch WiFi Apple iPad Pros on sale for up t...
B&H Photo has 12″ WiFi Apple iPad Pros on sale for up to $50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off MSRP... Read more
9-inch Apple WiFi iPad Pros on sale for $20-$...
B&H Photo has 9.7″ Apple WiFi iPad Pros on sale for $20-$50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 256GB WiFi iPad Pro: $779.95 $20 off MSRP... Read more
Apple refurbished 2015 15-inch MacBook Pros a...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available starting at $1699. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.2GHz Retina MacBook Pro... Read more
Back in stock! 13-inch 2.7GHz Retina MacBook...
Apple has Apple Certified Refurbished 2015 13″ 2.7GHz/128GB Retina MacBook Pros (MF839LL/A) available again for $1099 including free shipping. That’s $200 off MSRP, and it’s the lowest price... Read more
Slate Tablet Market Share to Fall Below 75% i...
After two years of decline, the tablet market is showing signs of new life as productivity trumps entertainment, and tablets become cost-effective computing devices for both entertainment and... Read more
Yostand Launches Indigogo campaign for iStand...
China-based startup Yostand (meaning ‘your stand’), has announced the launch of its Indigogo campaign for their newly awaited iStand7. This product is a one of a kind iPhone battery case that offers... Read more
Green App – Budget Forecasting Now Available...
Indianapolis, Indiana based CoopToons has announced the release of Green – Budget Forecasting 1.5, an update to their personal budgeting app developed exclusively for iOS devices. Green aims to be a... Read more

Jobs Board

SW Engineer *Apple* TV Frameworks - Apple I...
The Apple TV team is looking for a software...create features that reflect the look and feel of Apple TV. Description: Were looking for someone who is Read more
Hardware Design Validation Engineer - *Apple...
The Apple Watch team is looking for a Hardware Design Validation Engineer. This person will be part of the Apple Watch hardware team with responsibilities for Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 53586123 Pittsburgh, Pennsylvania, United States Posted: Nov. 28, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC Read more
*Apple* Retail - Multiple Positions- Plano,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.