NFS and SMB – Do They Support Sparse Files?

nfsserver-message-blocksparse-files

This question was previously asked in stack overflow but the good folks there have recommended that i try the community over here instead.

I am researching on sparse files with regards to various filesystems and am trying to find something concrete that states that sparse files is supported by Network File Systems (NFS) or Server Message Block (SMB).

I understand that SMB is widely used in Windows and that according to this entry, an SMB server can support sparse file even if the underlying file system does not. However, if i am right, the file system that does not support sparse files would just fill the 'holes' with zeroes and this could lead to a performance problem.

With regards to NFS, i have not been able to find out anything about using NFS supporting sparse files.

Hence, my questions is,

Are sparse files supported in NFS and SMB ?

Best Answer

NFS: it has a partial support for sparse file. Basically, it supports creating a sparse file but, when reading, the file is expanded to include zeroes. This means that, while you can create a sparse file via NFS, when reading back that very same file the in-transit network data will include any zeroes found on the original file. A simple test show that behavior:

cd /mnt/nfs
truncate test.img -s 1G
ls -lh test.img

-rw-r--r--. 1 root root 1.0G Oct 26 11:29 test.img

du -hs test.img

0 test.img

As you can see, the test.img file has an on-disk size of 0 bytes. However, reading back it using dd if=test.img of=/dev/null bs=1M iflag=direct shows

1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 10.2269 s, 105 MB/s

It is clear that when transferring the sparse file, it is expanded to include all zeros.

NFSv4.2 will expand by including special handling for network transfer of sparse file. In other word, with NFSv4.2 the above dd will complete almost instantly.

SMB: it has the same behavior as NFS, at least in my test environments, using a Samba v3.6.x server with CIFS v1 and a Linux client using mount.cifs. Maybe under Windows it behave differently...

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