How do I mount sub-directory to a hard drive in Linux?

Let us assume that I have two hard drives (A,B) and have the following directories:

• /var/www
• /var/www/upload

Currently if I upload a file to /var/www OR /var/www/upload ; it will be saved in drive A.

How do I make the folder /var/www/upload point to the drive B. So if I upload a file to /var/www/upload it will be saved in drive B but when I upload a file to /var/www ,it will be saved in drive A.

I assumed that disk A is not mounted as the root (/) filesystem. If it is, just ignore lines with driveA.

Edit your /etc/fstab:

/dev/diskA    /var/www/          auto   defaults    1   2
/dev/diskB    /var/www/upload    auto   defaults    1   2

You can replace "auto" by the filesystem you have on that partition, but the above should work anyway.

If disks A and B are mounted elsewhere you can try symlinking:

ln -s /path/to/driveA_mountpoint /var/www/
ln -s /path/to/driveB_mountpoint /var/www/upload

Note: /var/www and directory "upload" on driveA must not exist or this will fail.

Personally I prefer using the bind option of mount:

mount -o bind /var/www/ /path/to/driveA_mountpoint
mount -o bind /var/www/upload /path/to/driveB_mountpoint

Consider editing /etc/fstab though - it's probably the best way.

is Hard Drive B mounted? If it is,

ln -s /path/to/hard/drive/B/mount/point /var/www/upload

Otherwise

mount -t <fstype> -o defaults /dev/<hard driver B> /var/www/upload