Available Disk Space Discrepancy after copying files from HDD to SSD

[2frustrated]

Hi Everyone.

Apologies if this is the posted in the wrong place.

I have a 2TB Western Digital My passport HDD and have copied all files to a 2TB SanDisk Extreme SSD.

From Terminal, the WD drive details are as follows:

Device Identifier: disk4s1
Device Node: /dev/disk4s1
Whole: No
Part of Whole: disk4
Volume Name: My Passport
Mounted: Yes
Mount Point: /Volumes/My Passport
Partition Type: Windows_NTFS
File System Personality: ExFAT
Type (Bundle): exfat
Name (User Visible): ExFAT
OS Can Be Installed: No
Media Type: Generic
Protocol: USB
SMART Status: Not Supported
Volume UUID: 6BEC41AC-3CBE-3154-B062-EE077608F971
Partition Offset: 1048576 Bytes (2048 512-Byte-Device-Blocks)
Disk Size: 2.0 TB (2000364240896 Bytes) (exactly 3906961408 512-Byte-Units)
Device Block Size: 512 Bytes
Volume Total Space: 2.0 TB (2000301326336 Bytes) (exactly 3906838528 512-Byte-Units)
Volume Used Space: 1.2 TB (1171128320000 Bytes) (exactly 2287360000 512-Byte-Units) (58.5%)
Volume Free Space: 829.2 GB (829173006336 Bytes) (exactly 1619478528 512-Byte-Units) (41.5%)
Allocation Block Size: 131072 Bytes
Media OS Use Only: No
Media Read-Only: No
Volume Read-Only: No
Device Location: External
Removable Media: Fixed
Solid State: Info not available

And the SSD details are as follows:

Device Identifier: disk5s1
Device Node: /dev/disk5s1
Whole: No
Part of Whole: disk5
Volume Name: Extreme SSD
Mounted: Yes
Mount Point: /Volumes/Extreme SSD
Partition Type: Microsoft Basic Data
File System Personality: ExFAT
Type (Bundle): exfat
Name (User Visible): ExFAT
OS Can Be Installed: No
Media Type: Generic
Protocol: USB
SMART Status: Not Supported
Volume UUID: 4CE2FDB8-F540-3195-91AF-463C458FF4E9
Disk / Partition UUID: A36307F2-9978-46D0-BF5A-D6EA849A01DF
Partition Offset: 1048576 Bytes (2048 512-Byte-Device-Blocks)
Disk Size: 2.0 TB (2000363274752 Bytes) (exactly 3906959521 512-Byte-Units)
Device Block Size: 512 Bytes
Volume Total Space: 2.0 TB (2000353755136 Bytes) (exactly 3906940928 512-Byte-Units)
Volume Used Space: 1.2 TB (1213107011584 Bytes) (exactly 2369349632 512-Byte-Units) (60.6%)
Volume Free Space: 787.2 GB (787246743552 Bytes) (exactly 1537591296 512-Byte-Units) (39.4%)
Allocation Block Size: 1048576 Bytes
Media OS Use Only: No
Media Read-Only: No
Volume Read-Only: No
Device Location: External
Removable Media: Fixed
Solid State: Info not available

I would like to understand why the SSD is using 40 GB more space for the same files? Also attached is a screen grab of the same folder on both drives showing the different sizes on disk (1.4GB vs 1.95GB).

Your help is appreciated.

 

[Toutou]

Larger allocation block sizes make the filesystem less space-efficient but faster (less filesystem-level block accesses per file) and vice versa.

The allocation block size on your SSD's ExFAT filesystem is set to 1048576 Bytes, that's exactly one mebibyte (or roughly one megabyte, the better known misnomer).

That means that any file of any size, even smaller than 1 MiB, will, in the eyes of the filesystem, take up 1 MiB of storage, and all files, no matter how large, will always take up the smallest possible multiple of 1 MiB.

On your HDD the allocation block size is set to 131072, that's only 128 KiB, which means that very small files can be packed much tighter and files of any size will only waste < 128 KiB of storage.

e.g. a PNG screenshot of your post is 269 KiB. It would take up 384 KiB on your HDD, but 1024 KiB on your SSD.
Have a thousand of those and the difference is ~ 650 MiB.

 

[2frustrated]

Hi Toutou,

Thanks for the explanation and confirming everything is normal.

So in theory, I could make my HDD and SSD transfer files faster by decreasing the allocation block size but the trade off would be less available space.

Why would all drives not come formatted with the same allocation block size? Is this a sneaky way for manufacturers to show they are faster than their competitors? I assumed quoted drive manufacturer's speeds were a basis for an apples for apple comparison to be made.

If my SSD is transferring data at around 1GB/per min and I were to reduce the allocation block size to match my HDD what would be the impact on speed? For example would 1GB now transfer in 1 min 30s?