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I'm getting TM backups whose sizes don't match the target. My 260GB content on MBA SSD may produce a 220GB backup. My ~125GB IPhoto lib may produce a 95GB backup. Yet the other 30GBs of photos in the pictures folder back up to precisely the same size as on the SSD.

I've read TM does no compression. and I've had a tech support guy tell me that much difference (~25%) in the iPhoto lib is normal. But he hasnt explained why the size of the other pix (or music or movies, for that matter) isn't reduced. I'm left worrying My iPhoto lib isn't being backed up properly.

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4 Answers 4

No, it does not compress it, but the size of the library is in fact different:

The iPhoto Library is not a monolithic file, but a so called bundle (a folder disguised as a file). If you call file > Info (cmd-i), then it's size needs to be calculated, it's subfolder and files can be seen by the usual developer tools.

So, from the POV of time machine, or the file system in general, the iPhoto library is a huge folder with thousands of files.

Now comes another issue into play: Storage media are organized in file sectors of fixed size. The size a file takes on disk is not the number of bytes of this file, but the number of sectors which the file occupies by the number of bytes per sector. The sector size usually depends on the specific file system and media capacity.

Even a 1-byte file takes one sector, so on a hard drive the smallest file size might be 4 KB, on a flash drive 16 kB.

In consequence that means that the sector size of the medium you save to has a huge impact on the overall size of your iPhoto library. I take an educated guess that the sector size of your SSD is bigger than on your backup drive.

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It doesn't compress it, but it does leave out some non-essential data. In my case, the "iPod Photo Cache" and the file BigBlobs.apdb (this appears to be related to iPhoto-Aperture integration), both of which likely get regenerated automatically when restoring a backup:

$ sudo mdfind "com_apple_backup_excludeItem = 'com.apple.backupd'"|grep iPhoto
/Users/chucker/Pictures/iPhoto Library/iPod Photo Cache
/Users/chucker/Pictures/iPhoto Library/Database/BigBlobs.apdb

The sector size difference Kabe mentions may explain part of the discrepancy as well.

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+1. Most of the difference is likely due to the "iPod Photo Cache" which can become huge (larger than the library itself). –  Thilo Aug 8 '11 at 4:04
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It's worth pointing out that time machine does one FULL backup and every back up from then on is incremental, meaning only the differences since the previous backup are backed up rather than duplicating files. The iPhoto library may appear smeller in the backup because only a portion of it changed, and not the whole

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Browsing each backup snapshot individually will reveal the full data (thanks to hardlinks), though, not just incremental changes. –  Sören Kuklau Aug 7 '11 at 21:11
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Over time your iPhoto Library collects a whole pad of cruft, caches, thumbnail archives, temporary areas etc that can take up a large amount of space, but are not necessarily required when restoring your data, as it can/will all be recreated on the fly as and when you open photos etc. For this reason TM does not backup your entire machine (although it says it does) and has some inbuilt exclusion lists including these areas for iPhoto (and similar ones for iTunes too).

Basically, don't worry about it.

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