File system query.

[blal]

Hi friends,

I have a query .

We have a file system which is on internal disk mounted to /xxx directory . Now one another file system which is from SAN is mounted to /xxx/yy/zz directory .

/xxx has all appication and /xxx/yy/zz has data files.

Normally we mount all file systems to directories under root . Does this kind of a mounting (FS under another FS)gives any problems , like performace etc ..

Thanks in advance.
baiju.

[James A. Donovan]

It can certainly be done. I use that method myself for certain filesystems (/var/adm/crash comes to mind). The only issue I've ever encountered is that when you want/need to increase the size of the /xxx filesystem you ust first unmount the /xxx/yy/zz filesystem, even if you have OnlineJFS installed. I've never encountered any performance problems.

[Marco Santerre]

Performance wise .. no.. the problems you may encounter is if one day you actually forget, or have a problem mounting your second filesystem (/xx/yy) and you put data and then mount /xx/yy, you won't see your data as your file system is now on top of it.. and therefore, you might not know where the space is allocated.

[Helen French]

There won't be any performance impact. The issue is if you loose the first LV or if you encounter any problems with that, the second file system also will be affected.

You can also create link files that point to a file system mounted on root, instead of actual mounts.

[Jean-Luc Oudart]

If you mount manually the fs, I think you must do it in the order /xxx and then /xxx/yy/zz (umount : reverse)

regards,
Jean-Luc

[A. Clay Stephenson]

In the old days of UNIX, we were cautioned to avoid nested filesystem mountpoints because there was a small performance penalty associated with the extra level of indirection. Even then the hits were very small and now they are tiny. This is a practical approach to space management. The main consideration is to make certain that the filesystems are now mounted and unmounted in the correct order.

[Graham Cameron_1]

I don't think you will encounter any performance problems with this, we do it all the time.

However, it is conventional (but not essential) to use dedicated, empty directories for your mount points.

Otherwise, any files present in a directory on top of which a file system is mounted become inaccessible. In your case, if on your internal disk, there were files under /xxx/yyy/zz, then when you mount the SAN filesystem onto this dir, the files can't be seen (but they are still there).

Hope this helps. I attach a mount -pl listing to show how we do it.

-- Graham