Sparse files
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Create sparse file
truncate -s 10G ./file
Copy locally
cp
cp from to
dd
dd if=srcFile of=dstFile iflag=direct oflag=direct bs=64K conv=sparse
Show real size
ls
Show real size and logical file size:
ls -lhs file
du
Show real file size:
du -sh file
Copy over network
rsync
Without compression all data are transferred over network even the file is sparse.
rsync -aS --progress file user@host:/dir
With compression speed depends more on CPU performance as all data needs to be compressed. Sparse space is well compressed and transferred data greatly reduced. The data which can't be well compressed are transferred slower than without compression.
rsync -aSz --progress file user@host:/dir
Reclaim space on ext4 image
Mount old image as loopback device:
mount old.raw /mnt/old
Get size of the file:
ls -l old.raw
Create second sparse image file with the same size:
truncate -s <size> new.raw
Format it with ext4 file system:
mkfs.ext4 new.raw
Mount new image as loopback device:
mount new.raw /mnt/new
Synchronize entire disk:
rsync -aSAXv /mnt/old/ /mnt/new
Unmount disks:
umount /mnt/old
umount /mnt/new