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Old 07-06-2011, 10:01 PM   #1
jcd29
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Disk capacity discrepancy in ext4


I've been seeing this with every single ext4 or ext3 disk/partition I work with, but I decided to make a post about it now since I got a bigger hard drive.

I formatted a 1TB external HDD as ext4 last night.

This is what I got:

http://img146.imageshack.us/img146/2259/71163295.png

http://img69.imageshack.us/img69/7180/70755142.png

The first imaage is from GParted, and the second one from Nautilus, when you right click the disk and click on Properties. Notice how GParted says the disk is 931 GiB while the Properties window says it's 917 GB. This happens to every single ext4 or ext3 drive I've worked with, and every other Linux computer with ext4 or ext3 I've seen. I know about the 5% reserved space in ext4, but it's not that. The 5% would be that 45.8 GB you see as used (the disk in actually empty) in the Properties image.

Now, I know about how HDD manufacturers will tell you it's 1000TB when in reality it's not, etc. The thing is, according to this converter: http://wintelguy.com/gb2gib.html

1000 GB would be 931 GiB. So far so good! The GParted image shows the disk as 931 GiB. The thing is, as you can see, it also shows 14.81 GiB used right out of the gate, which leaves that 917 GiB that the Properties image takes as the "total capacity".

So what's going on here? Is it just that ext4 takes that much off the actual disk capacity? Because I compared with a friends 1TB external, and it showed its total capacity as 931 in both GParted AND the disk properties, but of course his disk was NTFS.

It'd be weird if ext4 takes off 14 GiB while NTFS takes nothing, so I'm wondering what exactly is that and if it can be changed/fixed somehow?

Like I said, this applies to every single ext4 and ext3 drive I've worked with.
 
Old 07-06-2011, 11:53 PM   #2
andrewthomas
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The reduction is probably due to journaling.

If you really want to, you could turn it off.

Quote:
2.9. "No Journaling" mode
Journaling ensures the integrity of the filesystem by keeping a log of the ongoing disk changes. However, it is known to have a small overhead. Some people with special requirements and workloads can run without a journal and its integrity advantages. In Ext4 the journaling feature can be disabled, which provides a small performance improvement.
http://kernelnewbies.org/Ext4#head-b...f00764cb9beb1e
 
Old 07-07-2011, 06:24 AM   #3
syg00
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Quote:
Originally Posted by jcd29 View Post
It'd be weird if ext4 takes off 14 GiB while NTFS takes nothing
My cynical view would be that FOSS will tell you it has used some (for journal or whatever) while M$oft don't.
Maybe it's just my years in the business showing through ...
 
Old 07-07-2011, 10:01 AM   #4
jcd29
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Is the journaling something that's safe to disable in external drives, or should I keep it? What could the consequences of keeping it be?

Quote:
Originally Posted by syg00 View Post
My cynical view would be that FOSS will tell you it has used some (for journal or whatever) while M$oft don't.
Maybe it's just my years in the business showing through ...
I dunno, I'd tend to agree but even when you format a 1 TB drive as NTFS in Linux, you see the size as 931 instead of 917, so I don't think it's that.
 
Old 07-07-2011, 10:25 AM   #5
Wim Sturkenboom
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syg00 is referring to the 14GB extra that you have 'lost', not to the 931GB
 
Old 07-07-2011, 10:29 AM   #6
jcd29
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Quote:
Originally Posted by Wim Sturkenboom View Post
syg00 is referring to the 14GB extra that you have 'lost', not to the 931GB
It's the same

The drive is shown as 917GB if it's ext4, but it's shown as 931GB if it's NTFS (both in Linux). That 14GB is what's lost because of the journaling I'm guessing.

I don't know if it's worth disabling though. If it gave a big performance boost maybe. But I don't know the downsides yet.
 
Old 07-11-2011, 09:06 PM   #7
jv2112
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Lightbulb



By default drives reserve 5% of the drive for root to write to. You can reduce that by using tune2fs -m # /dev/sdxx .


Quote:

man tune2fs

-m reserved-blocks-percentage
Set the percentage of the filesystem which may only be allocated
by privileged processes. Reserving some number of filesystem
blocks for use by privileged processes is done to avoid filesys‐
tem fragmentation, and to allow system daemons, such as sys‐
logd(8), to continue to function correctly after non-privileged
processes are prevented from writing to the filesystem. Nor‐
mally, the default percentage of reserved blocks is 5%.


 
  


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