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Journaling Bits

Volume Number: 21 (2005)
Issue Number: 6
Column Tag: Programming

Journaling Bits

A Look At The OS X "Personal Diary"

by Brad Belyeu

Welcome

My typical work day is filled with interruptions. Phones constantly ringing; people asking questions, etc. Everyone wants a piece of my time. Interruptions are not only a major hit to productivity and effective time management; but when you're trying to replace a PowerBook logic board, it can mean bigger problems. They sometimes leave me wondering, "Where did I set my screwdriver?", or "Where does this screw go again?" Now imagine having a secretary that follows you around all day recording what you do. Whenever you finish a task, it is scratched off a list; and whenever you start a new task, it is added to a list. Anytime you had an interruption, you could simply ask your secretary what you were doing before the interruption. That would be amazing! This is what Apple's software engineer's were thinking when they introduced file system journaling.

Meet The Secretary

To understand why file system journaling is important, we need to understand file systems & take a look at OS X systems prior to journaling. File systems are like library cataloging systems. Library catalogs can tell you an exact name & location of any book in the library; just like file systems can tell you the exact name & location of a piece of data on a drive. Both systems regulate naming & location conventions for what they store. If a file system gets corrupted, it would be like trying to read a library catalog in a language you can't understand. Journaling was introduced into OS 10.2.2 Server edition and into the client in 10.3. In previous OS versions, when you have any of these failures, they leave your system volume in an unknown state because there is no record of where the disk was interrupted. The OS then had to do a data integrity check of the entire disk. Interruptions can also damage system files rendering the boot volume unusable. This is usually when you restart and are presented with a folder and the infamous flashing question mark because the computer can't find a disk to startup. But when journaling is enabled, the computer tracks file system operations and immediately logs them in a journal. When restarting after a failure, the operating system can use the journal to return the files system to its previous state. This eliminates the need to check the entire drive for inconsistencies. Instead, when the computer is restarted, it takes a look at its journal, reads to find out what it was doing, and picks up right where it left off. File system journaling is a major advancement in protecting the file system from power outages, hardware failures, or software crashes.

The Good

A system continuity failure interrupts the read & write processes of a hard drive. These interruptions create discrepancies between the file system (library catalog) and the actual location of the stored files (location of books in library). In a journaled file system the OS has a record of disk activity, preventing the disk from becoming unusable (most of the time) which also prevents repairs that cost both time & money. It also cuts down on the time it takes to restart the computer after one of these failures because the entire disk doesn't have to be checked for inconsistencies block-by-block. Although a multi-gigabyte hard drive only takes several minutes to check, the original idea in introducing file system journaling was to minimize down-time for servers because a multi-terabyte disk could take hours to check. A journaled file system takes just a few seconds to return to operating condition no matter what the size of the drive is.

File system journaling was developed by Apple to be an extension to the current file system. This is good news because it lets you enable/disable journaling without reformatting the disk! The Mac OS Extended volume format (HFS+) can change between journaled and non-journaled without any data loss. The advantages of this will become clear later in the article as we discuss reasons you may not want to use file system journaling.

Because the journal works at the system level, it is invisible to the applications on the drive. Other than some disk utilities, all applications and network setups are compatible with journaling. Most 3rd party disk utilities will also work with journaling. Just in case you were wondering, finished actions are removed from the journal so the journal file doesn't become too large. Just like that the secretary crosses completed tasks off your list and throws them in the trash.

Journaled file systems are backward compatible with the regular HFS+ file systems. Meaning that computers using an older Mac OS can have full access to journaled volumes. This is great news for external hard drive users with multiple computers. That way you can use journaling on your hard drive and all your Macs can use the drive whether they are running OS 10.3 or an earlier version.

The Not So Good

Journaling doesn't come without its cost, which happens to be performance. But fortunately, the performance hit only comes when the system is writing to the disk and not when it is reading from the disk. I've never been able to actually tell the difference in speed on drives running journaled verses non-journaled drives. Supposedly, if you have a client version of the OS, the performance hit can be up to 20%, but the server version of the OS comes with a buffered version of journaling. If you have enough RAM, the server version is only supposed to take a 2-3% hit on hard drive performance.

Here's my general opinion on two occasions when you shouldn't use journaling. If you have a drive that is primarily set up to be read-only access, you don't need journaling. For example, if you have a hard drive that you keep your applications on (or just the OS), but not your actual documents or working files on, then you probably don't need file system journaling. This is because you are just pulling data off the drive 99% of the time and are rarely saving data back to the drive. If your system fails, your drive probably wasn't in the middle of writing data. Or if you have a file server disk that is always under heavy speed demands AND you back it up on at least a daily basis, I would consider disabling file system journaling. This might be the case if you are storing large data files containing audio or video that are accessed frequently.

Although file system journaling will help you save data that has already been journaled to your drive; you may still lose data in the event of a failure that is in the buffer before it is written to the drive. So file system journaling definitely needs to be complimented by a battery backup system.

The last drawback to consider is the failure of some 3rd party disk utilities to work with file system journaling. Make sure you check compatibility on a manufacturer's website before using a 3rd party utility on a journaled file system. Many of the vendors released updates specifically for their product to work with a journaled file system. I've heard nightmares of people losing all their data by running an incompatible disk utility with a journaled file system.

File system journaling has its limitations and is not a fix-all answer to your data problems. According to Apple, file system journaling is one of several measures you should take to avoid data loss. File system journaling should be coupled with RAID (redundant array of indexed disks) storage, a continuous backup strategy, and an uninterrupted power source (UPS) for maximum protection of you digital assets.

Enabling/Disabling

You can enable or disable file system journaling using Disk Utility (located in the Applications/Utilities folder). Select a volume in Disk Utility (figure 1).


Figure 1.

If you want to enable journaling, you can simply click 'Enable Journaling' from the pane. This will enable journaling without erasing data on the disk. If you'd like to disable journaling, you can select the volume and then click File-Disable Journaling from the menu bar.

You can also enable or disable file system journaling using the command line version of Disk Utility, diskutil. To have sufficient privileges you'll have to use sudo (super user do) with diskutil. To enable file system journaling on the root directory type:

sudo diskutil enableJournal /

and then type in your administrator password. If you'd like to disable file system journaling on the root directory use:

sudo diskutil disableJournal /

followed by your administrator password. To enable or disable on volumes other than the startup disk replace / with /Volumes/(VolumeName).

Repairing Journaled Drives

To repair an ailing journaled drive you can once again use Disk Utility. The trick is that you can't repair the disk you booted from; so if you want to repair your startup volume with Disk Utility, you have to startup to a CD or another drive. Once you have Disk Utility open, select the drive that you want to repair and then click 'Repair Disk'. If you don't have another drive you can startup from you'll be better off using a 3rd party utility or the command line.

If you don't want to spend the money on a 3rd party disk utility, try using the command line utility fsck_hfs. To force fsck_hfs to check a journaled volume you need to use the argument -f. Also, if you just want to verify status of the drive, use the -n flag; but if you want to repair the drive, use the -y flag.

There are a number of excellent 3rd party disk utilities. My favorite are DiskWarrior X & TechTool Pro. There are also a number of shareware & freeware applications to be found just by searching versiontracker.com or macupdate.com.

Wrapping up Journaling

Using file system journaling is a great idea for most users. There are the occasions to disable it, but for the most part it makes our lives easier. That is where the analogy leaves the library catalog. File system journaling is done automatically, but looking up a book in a library can sometimes be challenging. I'm just glad file system journaling keeps track of everything without my lifting a finger. But remember, file system journaling does not excuse you from needing a regular system backup!


Brad Belyeu is the President of ABConsulting based out of Oklahoma City, OK. He is an Apple Certified Technician and a certified member of the Apple Consultant Network. Most importantly, he is happily married.

 
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