One common suggestion to improve Lightroom’s performance is to disable the automatic writing of changes into XMP files. But what does that even mean, and do or not do? If having this option on is detrimental to the performance, what are the benefits?

Basics

Lightroom is a completely non-destructive photo editor. Any edits that you make to a photo are saved as a set of instructions (like a recipe) into Lightroom’s catalog. Lightroom interprets the instructions and applies them to the photograph when you view it in Lightroom. But only when you Export the photo, Lightroom actually renders a new file, with the edits permanently applied. Your original is never changed.

This makes the Lightroom catalog the most vital file – every edit that you make, every keyword that you apply, rating you give, is stored in the catalog. In addition to the catalog though, Lightroom can write this instruction set to an external XMP “sidecar” file (in the case of proprietary raw data formats such as NEF or CR2), or embed the XMP right into the photo as a data block (in the case of TIFF, JPEG and DNG files) – but it needs to be told to do so.

Writing the changes into individual files in addition to the catalog increases the disk access, and that of course affects performance. How much depends on the type of disk of course – an SSD drive or a desktop hard drive spinning at 7200 rpm are naturally faster than a small laptop hard drive spinning at 4200 rpm, for example.

On the plus side though, with automatic writing of changes into XMP enabled, you will have an instant backup of all changes to a photo in Lightroom (develop settings, metadata). What’s not saved are the pick/reject flags, and catalog-specific features like Virtual Copies, and stacking. Nevertheless, depending on your backup strategy, this may be lifesaver! I have two examples below but first, here’s where to find this setting:

Open your Catalog Settings and click on the Metadata tab, then check or uncheck the box highlighted below:

In italics, you see a warning message in the dialog box when the setting is disabled: Changes made in Lightroom will not automatically be visible in other applications unless written to XMP. Those “other applications” will have to support XMP of course and for most of us, that will mean Photoshop and Adobe Camera Raw (ACR): when “Automatically write changes into XMP” is enabled, those can read your edits and other metadata (Bridge will read the keywords too, for example – but if you’re a Lightroom user, why on Earth would you also use Bridge?;-).

Following are two scenarios where the “instant backup” benefit of having the changes written to XMP (would have) saved the day – and both actually happened.

Catalog Corruption

This happened to me! My backup script runs every night, usually at a time when I’m still awake and/or at the computer, so I know it’s running. Now one of my hard drives was beginning to fail, but there wasn’t just a read error, no – the entire system shut itself down without warning when it tried to read certain files from that drive (it was an SSD). It’s a strange behavior for a failing drive (and computer system), and perhaps quite rare too – but nevertheless, it happened. So instead of backing up all files as scheduled, my computer shut itself down when the backup software tried to access certain files. The photos were safe, they’re on a different drive – but I was without a Lightroom catalog backup of that particular day’s work.

When I turned on the computer the next morning I noticed that something was wrong – because it wasn’t waking up from hibernation, but did a cold boot. Huh? I saw that the backup wasn’t complete, but because of the instant shutdown, there was nothing in the Windows event logs that would have given me anything useful (besides the usual “the system was shut down unexpectedly” that is only added after you restart the machine).

Worse though, the drive had deteriorated further – I couldn’t read my Lightroom catalog from it anymore. Panic! I tried copying the Lightroom catalog to a different location, and about halfway through, the machine shut itself down again. Ouch! So I removed the drive and restored what was necessary from backup – including the last Lightroom catalog backup, from the night before.

To get the missing files back into Lightroom, all I had to do was to synchronize the parent folder (again, the photos are on a different drive than the catalog) – but thanks to the XMP backup files, all my edits were preserved too! The only thing missing was the flag status (pick/unflagged/rejected) because that isn’t stored in XMP files – easy to fix because every photo that has a rating is a “pick” in my workflow.

I know this is unusual but we’re talking about computers and you know the saying: “the question isn’t IF a hard drive will fail, but WHEN.” Here’s another, and far more ordinary example:

Accidental Removal of a Folder

One of my clients accidentally removed an entire folder from Lightroom – with plenty of fully edited images from a portrait session. This happens more often than you’d think – in the context menu that opens when you right click a folder, the options Rename and Remove are right next to each other! There’s a warning dialog box of course, but you know how easily such a dialog box is confirmed when you want to get something done – “just get out of my way, machine!” … <clicks okay>

It was easy enough to get the photos back into Lightroom (Import -> Add), but all the edits were missing because they were only stored in the catalog (from which the folder had been removed). Hours of work – gone. This is where another safety mechanism that’s built into Lightroom helped: the catalog backups that are set to run once a week by default. We were able to open the backup catalog where the folder was still there. Its location on disk had changed but once updated, we wrote the metadata into XMP, re-opened the current catalog, and read it back in from the files on disk.

All edits were preserved, but the necessary steps were far from trivial, of course. Had “automatically write changes into XMP“ been enabled, adding the folder back into Lightroom would have been enough (Lightroom reads and interprets existing XMP metadata upon import).

Backup regularly, backup often

Looking at the bigger picture, both cases illustrate the importance of frequent backups, of course. Going back and having to redo the edits would have cost a lot of time. I think the tip to turn this feature off for better performance alone should not come without a warning.

Take care, backup regularly, and make sure that your backup is complete!

Further Reading

• Reducing the Lightroom catalog size (this will also reduce the size of the XMP sidecar files)
• DNG conversion and backup considerations