When migrating all the files from my Synology diskstation to OMV, on OMV more disk space was used, than I had expected, although both systems use exactly the same type of HDs (WD red, 6 TB) and both use ext4. But the pretty filled up disk on the diskstation (about 200 GB empty) did not fit onto OMV. I deleted some stuff, reformatted on OMV (because it was easy at this point) and tried again. Copied all the files with either cp -a or rsync to two different disks (same type), same result.
On OMV I have:
1M Blocks/used/available
5678073 5375011 16904
On diskstation
5673389 5375355 297935
Please note: Used is almost identical (as expected). On diskstation used + available sum up to almost exactly the disk space (99 MB difference), while on OMV there is a really huge difference of almost 300 GByte. This means, OMV uses over 5% more storage for the same files.
Why does OMV use a significant higher amount of storage under very comparable conditions? Obviously I am missing something significant.
I compared the output of tune2fs -l for the involved filesystems for OMV and diskstation. Following notable results/differences (highlighted by bold):
OMV
Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file
Filesystem flags: signed_directory_hash
Diskstation
Filesystem features: has_journal ext_attr resize_inode filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags: unsigned_directory_hash
Both:
Block size: 4096
Fragment size: 4096
Indode count / blocks count and other counts are very similar with one outstanding difference
OMV Reserved block count: 73256512
Diskstation Reserved block count: 25600
One other strange number. OMV reports: Lifetime writes: 2678 GB, while certainly over 5000 GB were written.
These file sysytems had quite some small files. On other file systems where large files use almost all capacity, there was almost no difference in storage usage between diskstation and OMV.
Do you have any explaination for the seemingly higher storage usage of OMV under comparable conditions? Actually, the diskstation volume was in use since about 1.5 years with lots of overwriting, deleting, new files and so on. The OMV volume was just copied once. In limited tests, both systems seem to perform well/as expected. The diskstation is often a bit slower. No surprise, as it is rather low end with ARM processor, and sometimes I see it CPU-bound on diskstation.