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Surprisingly, this is not easy to accomplish. In order to have some files on one storage system and others on another, you either have to use two different filesystems (and volumes and volume groups) and move the files with a script or SRM package, or you have to use an HSM (Hierarchical Storage Management) product.
One free method I've seen is to use a script to move the old files to your nearline/ATA storage and leave Unix symbolic links behind. Let's call this manual HSM. This works surprisingly well in some circumstances, especially if there are relatively few files to be managed in this way. But watch out -- each symbolic link requires an inode in the filesystem, and you can quickly run out of inodes! df_vxfs will show you how many inodes are left, and if you will have more than 8 million files, you need to use the "largefiles" option of mkfs when creating the filesystem. And look out for applications that aren't friendly to symbolic links, or none of this will work!
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