How can I keep my indexing progress when backing up my photos library from iPhone?

The only way to transfer all the information that is in a Photos Library is to use iCloud Photos. When you synchronize with iCloud Photos, then the Photos Libraries have exactly the same pictures, including all the information in the Photos database, which is more than is kept in any other metadata transfer. All my albums and folders are exactly the same on my iPhone, my iPad, my Mac, and at iCloud.com. Some devices don't do everything-- portable devices don't show titles or Smart Albums, for instance, and neither does the browser view at iCloud.com, but it's all there for transfers to a device that does show them.

Keep in mind, though, that when you delete pictures on your iPhone, if it's connected to iCloud.com, then those pictures will be deleted at iCloud.com, they'll be deleted on the Mac, and they'll be deleted everywhere else. A Photos Library can be archived by saving the Library that's on the Mac to an external drive that isn't connected to iCloud.

People often use "Optimize Storage" to save space. In that case, all the information isn't stored on the device, but it is stored at iCloud.com. If you are after an archive like backup, then you can't use "Optimize Storage" on your Mac, because that won't keep all the information. All the information will be available to the Mac through iCloud, but "Optimize" doesn't serve as a backup. If you don't have enough space on the Mac for a full backup, then you can use an external hard drive to store the Photos Library. You can see this:

Backup iCloud Photos with an Optimized Mac - Apple Community

Optimize storage can be very useful on a phone. My phone's optimized Library takes up less than 10% of the space that the full Library uses at iCloud.com. That's great! But I don't use Optimize on my Mac, because I want to have a real backup. iCloud.com isn't really a backup, because it not only copies pictures, it copies all your mistakes. You can't count on iCloud alone as a backup.

We can help with more details if you need it…

I am adding all the names to the People&Pets album in Photos on my Mac. Most of these names are synced with iCloud Photos to my iPhone. Just wait and see - sooner or later you should be seeing named faces on your Mac as well.

But Photos will repeat all indexing and scanning after reinstalling the system.

Why do you want to wipe the iPhone? Do you want to downgrade to an earlier system version?

Richard.Taylor wrote:

Face names won't pop up immediately, but my devices seem to all agree. The method it uses is proprietary, and Apple's not telling, but I doubt that it uses the IPTC names for this. It uses the information in its database that generated those IPTC names.

It should be possible to export Faces as .xmp from Photos library and then use exiftool to more permanently copy them to XMP (and IPTC) with exiftool.

It seems some macOS update has nuked faces from my current Photos library so I ran a test with an older sample from 8/2022 (I believe the .xmp format has stayed the same):

In the beginning the image.jpg has no metadata but the sidecar image.xmp has a list of faces (that quoted -sep '; ' option is to verify that they are indeed a separate list, not just a string. Change that to -sep '//' so the output changes accordingly):

exiftool -a -G1 -s -sep '; ' -Keywords -Subject .
======== ./image.jpg
======== ./image.xmp
[XMP-dc]        Subject                         : Name1; Name2

Then copy faces from .xmp to same name .jpg and .heic XMP and IPTC tags, and verify the result (the dot at the end of the command means "this folder" so all same name image and .xmp pairs are processed in that working directory):

exiftool -m -P -overwrite_original -TagsFromFile '%-.0f.xmp' '-XMP-dc:Subject<XMP-dc:Subject' '-IPTC:Keywords<XMP-dc:Subject' -ext jpg -ext heic .

exiftool -a -G1 -s -sep '; ' -Keywords -Subject .
======== ./image.jpg
[IPTC]          Keywords                        : Name1; Name2
[XMP-dc]        Subject                         : Name1; Name2
======== ./image.xmp
[XMP-dc]        Subject                         : Name1; Name2

edit: IPTC is an old and not very flexible format and I now remembered that you need to set UTF8 coding so also umlauts will copy correctly:

exiftool -m -P -overwrite_original '-IPTC:CodedCharacterSet=UTF8' -TagsFromFile '%-.0f.xmp' '-XMP-dc:Subject<XMP-dc:Subject' '-IPTC:Keywords<XMP-dc:Subject' -ext jpg -ext heic .

Or just leave IPTC copy off because current apps should read or prefer XMP anyway so keeping XMP and IPTC in sync is not needed anymore.

Also GraphicConverter can copy faces from .xmp to image keywords.

Face names are included in IPTC, and thus Exif, but as far as I can see, not the locations of the faces. Many of us include the names in captions and keywords, so that's just redundant. As far as I can tell, the only way to keep the information used to identify the faces is by transferring the actual database.

Making keywords of the face names is clever. I do that manually now, of course. In Photos you just select all in a Person Album, and add the keyword to all at once. It would be nice if this happened automatically, though.

Albums aren't things that pictures are in-- they're just lists of pictures you'd like to see together. So an album is just another set of bits, and they sometimes copy through before the pictures, sometimes after, sometimes just in there somewhere. No need to worry, yet.

Faces require a re-scan by the local database, and they can take a week or more to complete. That's the same with other attributes that require scans.

Photos continuously scans for face attributes to match with names. It seems to make decision using both new scans and previous data. Each device may come up with different results. How it does this changes with the OS. Face names won't pop up immediately, but my devices seem to all agree. The method it uses is proprietary, and Apple's not telling, but I doubt that it uses the IPTC names for this. It uses the information in its database that generated those IPTC names.