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 Post subject: SSD - How much free space to leave on drive?
PostPosted: Sun Jun 10, 2018 8:58 pm 

Joined: Sat Dec 21, 2002 2:18 am
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Location: Auckland New Zealand
I recently upgraded my MacBook Pro with a 1TB SSD for the system drive. I know that conventional wisdom says to keep 20% disc space free for swap files and under-the-hood operations. The 20% free rule goes back years and years, to when much smaller drives were the norm.

I’ve filled this SSD to 950GB with my system and sound libraries etc, is leaving 50GB free for swap files enough in this day and age?

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Sun Jun 10, 2018 11:03 pm 

Joined: Wed Jun 25, 2014 11:34 am
Posts: 6994
Pitch Black wrote:
… a 1TB SSD for the system drive.…

I’ve filled this SSD to 950GB with my system and sound libraries etc, is leaving 50GB free for swap files enough in this day and age?


Not really. SSDs are generally much better at having less space, bit this comes at a price. You need to leave quite a bit free space on your SSD or its write performance will considerably slow down.
When an SSD has plenty of free space, this means it has a lot of empty blocks too. When a file is written the data goes into the empty blocks.

When an SSD has little free space, it has a lot of partially filled blocks. When a file is to be written to storage, these partially filled blocks will have to be read and modified with new data and then written back. Which adds time to the write operation.

Ananadtech suggests even more free space here in Exploring the Relationship Between Spare Area and Performance Consistency in Modern SSDs.

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Mon Jun 11, 2018 2:28 am 

Joined: Wed Oct 24, 2007 4:50 am
Posts: 3821
in real-world terms the amount of free space impacted spinning hard drives a lot. with SSDs, there is a relationship between free space and consistency of write performance, but even so the performance is still far above what any spinning drive can achieve. so the tradeoff between space and speed is less critical in real-world usage.

i have run the 1TB SSD in my 2014 MacBook Pro at 2–5% free space for about 4 years now. there is a performance sacrifice, but it's worth it to not have to carry an external drive everywhere i go. i do have some big sample libraries on external drives, but all my main stuff is on the internal. i'm assuming that your drive is SATA though, and not a faster PCIe drive like my stock one.

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Mon Jun 11, 2018 6:19 am 

Joined: Wed Jun 25, 2014 11:34 am
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We can also note that different models of SSDs react differently to fuller disks. I've run one machine with a at times 90% full Crucial SSD for multiple years, but I've used the built-in cleaner functions — by letting it hum without being booted or doing any work for 12 hours or so - and I've used trim, both which certainly did help when this slowed down to molasses a year ago or so.

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Mon Jun 11, 2018 7:52 pm 

Joined: Sat Dec 21, 2002 2:18 am
Posts: 6410
Location: Auckland New Zealand
Thanks for your responses!

Sooo... if write speed is the thing that can potentially be compromised, I wonder how much of a problem this would be in the real world? For my own use-case the only time I'm writing a LOT of data to disk would be during a multi-track recording session of a drum kit or a full band. Most times i'm just writing one or two audio streams at a time, and its reading from disk (multi-track playback) that I use the vast majority of the time. Please enlighten me if I'm wrong.

I guess my main concern would be shortening the life of my SSD by constantly re-writing over the same sectors/cells as that remaining 50GB becomes my "working area".

All wisdom/folk-knowledge gratefully received!

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Mon Jun 11, 2018 9:48 pm 

Joined: Wed Oct 24, 2007 4:50 am
Posts: 3821
for your real-world application, i don't think you need to worry. even writing multitrack audio is very little data in SSD terms...

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 Post subject: Re: SSD - How much free space to leave on drive?
PostPosted: Mon Jun 11, 2018 10:14 pm 

Joined: Wed Jun 25, 2014 11:34 am
Posts: 6994
Pitch Black wrote:
…I wonder how much of a problem this would be in the real world?!


Unless your model is one of those susceptible to this, mostly this concerns smaller disks, likely it will be years before you see any problems. Trim active certainly helps in preventing this and is natively supported since Sierra also with third party drives. It just needs to be turned on.

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