When does Linux do SSD garbage collection?

Question

I'm trying to get an understanding of when my computer does garbage collection and frees up previously used blocks on the SSD.

Is this a task handled by the OS or the SSD controller?
Does this happen when I delete a file (permanently from trash), or does it wait until I try to overwrite that block?
How does a journaling filesystem affect this drive's performance and maintenance?
Does the system do anything special when my computer is idle and the screen is locked?
How can I tell if my firmware and/or kernel support the TRIM command?

For what it's worth, I'm using Ubuntu 9.04, 64-bit (kernel version 2.6.28-15-generic) and full disk encryption with LUKS. The SSD is a Dell 256GB with a Samsung controller with an ext3 filesystem for root and ext2 for boot. No other hard drives, no other OSes.

I've read through the AnandTech article talking about TRIM and Intel and OCZ and pages and blocks, which is what prompted my question in the first place - I have no idea where my own setup falls in this analysis. While I'm interested in the answers to the above questions, I'm more interested in learning how to answer these questions, if that makes any sense.

Answer 1

There is a new article from Anand with more info about SSD magic :) I'd advise you to read it too. TRIM is explained in more detail there. In general, it says that trim is good, but:

...Unfortunately, no drives properly support TRIM yet...

That support is only in beta firmware at the moment. So, if both your OS and controller firmware support TRIM, then it will work approximately this way: your OS will send that command to the SSD controller once some file is permanently deleted. Whether it will be actually trimmed at that moment or some other time depends entirely on your drives firmware implementation.

If your OS doesn't support TRIM, then there is a chance that you can get a wiper tool from your drive manufacturer. You can read how that works here.

If your drive controller doesn't support TRIM, then maybe you can get an update or some beta version of it's firmware.

Hope that helps.

Answer 2

For what it's worth, I'm using Ubuntu 9.04, 64-bit (kernel version 2.6.28-15-generic) and full disk encryption with LUKS.

Actually you may not want to use TRIM with full drive encryption, as it will indicate which parts of the disk you're not using by TRIMming them! This may make brute-forcing the passkey simpler if the unused regions of the disk are completely emptied.