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Should leave 20% free on HD, what % leave free on SSD?

Luba

macrumors 65816
Original poster
Apr 22, 2009
1,443
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I've been told I should leave about 20% free on a HD for better performance, that a HD really slows down when it's close to full. What about with a SSD, does the same rule follow, should I leave 20% free, or perhaps with a SSD only 5% free is the rule to follow??
 
Nov 28, 2010
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You should leave room for the swap files and temporary files. I currently leave 33% percent free and and vow to not get below 10GB free HDD space.
 
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Luba

macrumors 65816
Original poster
Apr 22, 2009
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So follow the same rule for SSDs as you would for HDDs?

You should leave room for the swap files and temporary files. I currently leave 33% percent free and and vow to not get below 10GB free HDD space.
 
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Honumaui

macrumors 6502a
Apr 18, 2008
757
51
%50 free for boot and %40 free for storage is my theory :)

SSD the same as other HDDs ? you can fill more depending on use but I would not get in that habit for my cache drives I have noticed they stay full speed up to about %80 full but for scratch better I noticed to not go over the %60 or so ?

boot I am leaving lots so not testing more than the %50
 
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johnnymg

macrumors 65816
Nov 16, 2008
1,316
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%50 free for boot and %40 free for storage is my theory :)

ssd the same as other hdds ? You can fill more depending on use but i would not get in that habit for my cache drives i have noticed they stay full speed up to about %80 full but for scratch better i noticed to not go over the %60 or so ?

Boot i am leaving lots so not testing more than the %50

10+
 
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VirtualRain

macrumors 603
Aug 1, 2008
6,304
116
Vancouver, BC
If you never write large files to your SSD, except to perhaps install an App, don't worry about it. Lack of capacity impacts the garbage collection's ability to remap free blocks and maintain write performance. Read performance is not impacted in any way by spare capacity, or lack of it.

If you use your SSD's for data storage and you are consistently writing to them, I would recommend you shoot for 20% spare capacity or more. If you get down to 10%, start taking action to clear some space or add a new drive. Otherwise your write performance will suffer, particularly for the period immediately after writing a large file as the garbage collection works overtime to reorganize the free space. But chances are, if you're down to 10%, you're forced to take action anyway as you're running out of space to store stuff, not so much because your performance is impacted.

SSD's have little to nothing in common with HD's so don't let old-school HD thinking impact your SSD strategy.
 
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peskaa

macrumors 68020
Mar 13, 2008
2,104
5
London, UK
%50 free for boot and %40 free for storage is my theory :)

SSD the same as other HDDs ? you can fill more depending on use but I would not get in that habit for my cache drives I have noticed they stay full speed up to about %80 full but for scratch better I noticed to not go over the %60 or so ?

boot I am leaving lots so not testing more than the %50

Eh?
I have a 1TB drive as my Mac Pro's boot drive...why on Earth should I leave 500GB of that free? No swap file is ever going to be anywhere near that...
 
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Transporteur

macrumors 68030
Nov 30, 2008
2,729
3
UK
Eh?
I have a 1TB drive as my Mac Pro's boot drive...why on Earth should I leave 500GB of that free? No swap file is ever going to be anywhere near that...

That's true, but the performance of mechanical drives plummets after a certain rate. From my experience this is at about 70%. Especially for swap files, this means a pretty hefty performance hit.


SSD wise, my rule of thumb is 50 to 60%, depending on the drive size and machine I use. That always leaves enough space for swap and cache files.
 
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peskaa

macrumors 68020
Mar 13, 2008
2,104
5
London, UK
See, even 70% to me sounds very high - and because we're talking percentages, the actual amount of data is very variable. On my 1TB drive, that's still 300GB, whereas on an MBP 320GB drive that is 96GB. Throw in the fact that the amount of RAM you have also plays a key role, and it becomes a bit murky.

Anybody got any hard facts for drive space, rather than hearsay?
 
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philipma1957

macrumors 603
Apr 13, 2010
6,313
216
Howell, New Jersey
yes here are some tests for you

https://forums.macrumors.com/threads/1065243/


these are new 3tb hdd's note the partition into 1.5tb and the aja results. the outer half of the hdd runs at 120mb/s the inner half at 100mb/s


here is an explaination ;

so a hdd platter spins at 7200 rpm. but the outside edge is 10 inches in circumference the inner middle is 5 inches. both spin at 7200 rpm one moves 2x as fast because the circumference is twice as long. so if you partition a hdd in two parts the outer part is faster.

now lets say I do not partition the 3tb hdd, but never put more then 1tb of programs / data on it. it will keep the info on the outer rim so to speak and always be a bit quicker then if you use the inner portion of the hdd. So if you have the money to leave an hdd half empty it will be quicker due to simple speed of inner vs outer disc. I am not taking into account anything in this example other the the real speed of outer vs inner which is always the case with mechanical hdds.


As to the op question if you can afford to leave it half empty do so. If you can't afford to try for 1/4 to 1/3 empty. More true for an owc 240gb then an owc 200gb ssd. reason is the maker has left 40gb space empty so to speak on the 200gb owc ssd. I own a patriot inferno ssd 200gb type kind of a clone of the 200gb owc. I keep it about 140gb full. It works fine and I can't notice any slowdown at 140gb.
 
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peskaa

macrumors 68020
Mar 13, 2008
2,104
5
London, UK
Thanks, that makes perfect sense in terms of raw speed (ie: inner/outer platter differences), although leaving drives half empty is probably too pricey for me - particularly as I rip through drive space at a fair rate anyway.

Is there any data in terms of the OS though? Say you accept that you'll drop to the 100mb/s speed of the inner part of the platter, but is there a rough figure where you'll see the OS slow to a crawl etc? (In other words, the amount you should never go below?)
 
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Honumaui

macrumors 6502a
Apr 18, 2008
757
51
See, even 70% to me sounds very high - and because we're talking percentages, the actual amount of data is very variable. On my 1TB drive, that's still 300GB, whereas on an MBP 320GB drive that is 96GB. Throw in the fact that the amount of RAM you have also plays a key role, and it becomes a bit murky.

Anybody got any hard facts for drive space, rather than hearsay?

ram has nothing to do with HDD speed ?

but besides platter speed when empty its about data and when it fills and the location on platter and writing can suffer etc..

if you take your %70 full HDD and run some speed tests on it then clone the drive off to another drive and clean it off then test again


I have done this and found most cases a HDD over %60 full is about half as fast on avg :)
 
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philipma1957

macrumors 603
Apr 13, 2010
6,313
216
Howell, New Jersey
well on a 1tb hdd getting over 850gb will cause slowdowns that you will notice. more with big files then small ones. as a rule of thumb my biggest files are 17gb video files I like to have enough space on a disk for at least 5 more then the one i am recording. so i never go past 900gb on a 1tb. alot depends on what program if the hdd is also a strach disk. If it is an iTunes volume and you are only reading files. you can go up to 90 percent no problem at all.
 
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Luba

macrumors 65816
Original poster
Apr 22, 2009
1,443
240
I also have a SSD on a MBP which contains mostly (99%) apps. I am going to use iDisk to store the data files (mostly Office type files), but would download and/or keep files that I am working on, and when the project is done keep them up on iDisk. I am close to 20% now, I may install a few more apps that will bring me at or close to 20% . . . and of course will have a few data files around that wouldn't even amount to 50 MB. So with my MBP it's about 99% "reading" of apps, and 1% reading and writing of data. Sounds like since I am mostly using SSD for reading, I wouldn't be giving up any performance if I went down to 15% free or even 10% free??

If you never write large files to your SSD, except to perhaps install an App, don't worry about it. Lack of capacity impacts the garbage collection's ability to remap free blocks and maintain write performance. Read performance is not impacted in any way by spare capacity, or lack of it.

If you use your SSD's for data storage and you are consistently writing to them, I would recommend you shoot for 20% spare capacity or more. If you get down to 10%, start taking action to clear some space or add a new drive. Otherwise your write performance will suffer, particularly for the period immediately after writing a large file as the garbage collection works overtime to reorganize the free space. But chances are, if you're down to 10%, you're forced to take action anyway as you're running out of space to store stuff, not so much because your performance is impacted.

SSD's have little to nothing in common with HD's so don't let old-school HD thinking impact your SSD strategy.
 
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