Smart Update image size increasing over time

[benbean]

Apologies if this has been answered elsewhere, my search of the forums has been fruitless so far.

I backup nightly using Smart Update to an image file on a LaCie USB drive attached directly to my Mac Mini. As I understand it, the image should always mirror exactly the files on my drive (which currently total around 35GB).

What I see happening over time is the image file size keeps growing and growing. This morning for example it was up to about 75GB. If I delete the image and wait for the next backup to complete, it's down to 35GB or so again.

Whatupwiddat? Shouldn't the size be static?

Thanks!

[dnanian]

That's the way sparse image work, benbean -- Apple doesn't seem to recover deleted space very well. You can recover it by typing the following in Terminal:

hdiutil compact

(there's a space after compact), then drag the image file from Finder into Terminal. Then, press Return. It'll shrink the sparse image as much as it can.

Hope that helps.

[benbean]

Ok, useful tip. Thanks!

[jefman]

how big an operation is this "hdiutil compact"?

- does it require extra temporary space, or does it shrink it "in place"?
- is it relatively quick, or is there a lot of bit-shuffling going on?
- does it really recover most of the empty space, or just part of it?

basically i'm trying to figure out if SD is right for me. i like almost everything i've read about it, but needing a lot of "extra" space on the backup partition or image might be a show-stopper for me. my external firewire drive is usually very tight on space - i do video editing, so i don't need to back up these files but i need the space for working. i can't afford a backup sparseimage with 50GB of wasted space (or even 10GB for that matter).

i love the idea of being able to quickly do a "smart backup" every day. but if this needs to be followed by a lengthy "compact" operation (or it often fails due to not enough extra space available, requiring a full backup instead), it seems to defeat the purpose. is it really practical?

am i better off repartitioning my external drive (i'm a bit nervous about doing that operation), making a backup partition about the same size as my internal drive? or will this also routinely fail due to not enough extra space?

thanks... <jeff

[dnanian]

I honestly don't know how hdiutil compact operates, because I didn't write it, and don't have the source code. I'm certain that a lot of bit shuffling is going on... but it's not always necessary: I've never had to compact my own image, and I back up to it every night.

I think you're better off partitioning the drive. If it's mostly a scratch drive (and it sounds like it is), erasing it and partitioning shouldn't be a scary thing: it's quite simple to do!

Will it fail due to not enough space? In most cases, no. There are some situations where Smart Update can fail because you've moved around large files, and the order we "see" them shows us the new files before we find the files we delete... but it doesn't happen very often. (See the Troubleshooting section of the User's Guide for a description of this.)

Does that help?

[bradleebaker]

Ok great suggestions... my problem is my image resides on a xp machine. Will running this hidutil still work?

[dnanian]

Yes, it'll work over the network.

[bradleebaker]

Ok kewl.. I'll stop annoying now :)