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Today I saved a 90MB PSD file as (Photoshop) PDF file (using "Save As" in Photoshop CS6). Before and after saving, I checked the disk activity details in Activity Monitor on my Mac. It turned out that during the save process, Photoshop wrote 32GB to my disk and read 5GB from my disk. In the end the resulting PDF file was 110MB.

Out of curiosity, I was just wondering:

  1. Why converting a 90MB file to a 110MB file generates 32GB of temporary files; and
  2. Why Photoshop wrote 32GB of temporary files, but only read 5GB: why was the other 27GB written to my disk?

Addition: Let me clarify how I found these numbers (32GB/5GB). Activity Monitor on Mac shows information about processes, memory, network, and disk activity. The disk activity details include "Data read" and "Data written" (in GB). Before and after converting the file, I wrote those numbers down. By subtracting the first value from the last, I got the total data that was read/written during the conversion.

No other running processes were using my disk intensively. Before and after the conversion, the disk activity was very low. During the conversion, peaks were as high as 300MB/sec and sometimes it was writing a longer time at 200MB/sec (I have an SSD).

Activity Monitor The screenshot is not from my Mac, because it's set up with Dutch as primary language. However, you can understand what's displayed in this screenshot.

Furthermore, I saw the amount of available disk space in Finder shrink during the conversion. I don't know exactly how much space was left before the conversion, but it was around 30-50GB. Just before the conversion was finished, only 1GB of my disk was free (Mac also gave a warning about that). That means that 29-49GB was actually in use during the conversion.

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closed as primarily opinion-based by Xavierjazz, Sathya Jul 8 '13 at 4:12

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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I would contact Adobe with this question –  Ramhound Jul 8 '13 at 0:25
    
How did you determine that Photoshop wrote 32GB to your disk? I bet your method was incorrect. Perhaps you saw it reserve 32GB. –  David Schwartz Jul 8 '13 at 0:26
    
@DavidSchwartz I've added some clarification about it. –  Jonathan Jul 8 '13 at 9:07
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I suppose asking the vendor is a valid suggestion for this question –– and for maybe 25% of the questions we get on Super User. Why shouldn’t he ask here first? … … And I don’t understand why this question was closed as “primarily opinion-based”. How is this different from “Why do I need (large amount) of free space to edit a (much smaller) JPEG file?”, which can be answered by an explanation of image compression? –  Scott Jul 8 '13 at 16:46
    
Is someone going to review this question? I believe the question is thorough and well formulated (especially after my edit from 2 days ago). –  Jonathan Jul 10 '13 at 23:20