7. Modifying an Existing Installation

1. Q: Are linear MD's expandable? Can a new hard-drive/partition be added, and the size of the existing file system expanded?

   A: Miguel de Icaza < miguel@luthien.nuclecu.unam.mx> writes:

   I changed the ext2fs code to be aware of multiple-devices instead of the regular one device per file system assumption.

   So, when you want to extend a file system, you run a utility program that makes the appropriate changes on the new device (your extra partition) and then you just tell the system to extend the fs using the specified device.

   You can extend a file system with new devices at system operation time, no need to bring the system down (and whenever I get some extra time, you will be able to remove devices from the ext2 volume set, again without even having to go to single-user mode or any hack like that).

   You can get the patch for 2.1.x kernel from my web page:

   http://www.nuclecu.unam.mx/~miguel/ext2-volume

2. Q: Can I add disks to a RAID-5 array?

   A: Currently, (September 1997) no, not without erasing all data. A conversion utility to allow this does not yet exist. The problem is that the actual structure and layout of a RAID-5 array depends on the number of disks in the array. Of course, one can add drives by backing up the array to tape, deleting all data, creating a new array, and restoring from tape.

3. Q: What would happen to my RAID1/RAID0 sets if I shift one of the drives from being /dev/hdb to /dev/hdc? Because of cabling/case size/stupidity issues, I had to make my RAID sets on the same IDE controller (/dev/hda and /dev/hdb). Now that I've fixed some stuff, I want to move /dev/hdb to /dev/hdc. What would happen if I just change the /etc/mdtab and /etc/raid1.conf files to reflect the new location?

   A: For RAID-0/linear, one must be careful to specify the drives in exactly the same order. Thus, in the above example, if the original config is

   mdadd /dev/md0 /dev/hda /dev/hdb
               
   

   Then the new config *must* be

   mdadd /dev/md0 /dev/hda /dev/hdc
               
   

   For RAID-1/4/5, the drive's ''RAID number'' is stored in its RAID superblock, and therefore the order in which the disks are specified is not important. RAID-0/linear does not have a superblock due to it's older design, and the desire to maintain backwards compatibility with this older design.

4. Q: Can I convert a two-disk RAID-1 mirror to a three-disk RAID-5 array?

   A: Yes. Michael at BizSystems has come up with a clever, sneaky way of doing this. However, like virtually all manipulations of RAID arrays once they have data on them, it is dangerous and prone to human error. Make a backup before you start.

   
   Iive in dpersonality? 
   
   
   A:
   Yes!  Software RAID will work with any disk controller (IDE
   or SCSI) and any disks.  The disks do not have to be identical,
   nor do the controllers.  For example, a RAID mirror can be
   created with one half the mirror being a SCSI disk, and the 
   other an IDE disk.  The disks do not even have to be the same 
   size.  There are no restrictions on the mixing & matching of 
   disks and controllers.
         
   This is because Software RAID works with disk partitions, not 
   with the raw disks themselves.  The only recommendation is that
   for RAID levels 1 and 5, the disk partitions that are used as part 
   of the same set be the same size. If the partitions used to make 
   up the RAID 1 or 5 array are not the same size, then the excess 
   space in the larger partitions is wasted (not used).
   
   
   

5. Q: I have a twin channel BT-952, and the box states that it supports hardware RAID 0, 1 and 0+1. I have made a RAID set with two drives, the card apparently recognizes them when it's doing it's BIOS startup routine. I've been reading in the driver source code, but found no reference to the hardware RAID support. Anybody out there working on that?

   A: The Mylex/BusLogic FlashPoint boards with RAIDPlus are actually software RAID, not hardware RAID at all. RAIDPlus is only supported on Windows 95 and Windows NT, not on Netware or any of the Unix platforms. Aside from booting and configuration, the RAID support is actually in the OS drivers.

   While in theory Linux support for RAIDPlus is possible, the implementation of RAID-0/1/4/5 in the Linux kernel is much more flexible and should have superior performance, so there's little reason to support RAIDPlus directly.

6. Q: I want to run RAID with an SMP box. Is RAID SMP-safe?

   A: "I think so" is the best answer available at the time I write this (April 98). A number of users report that they have been using RAID with SMP for nearly a year, without problems. However, as of April 98 (circa kernel 2.1.9x), the following problems have been noted on the mailing list:

   • Adaptec AIC7xxx SCSI drivers are not SMP safe (General note: Adaptec adapters have a long & lengthly history of problems & flakiness in general. Although they seem to be the most easily available, widespread and cheapest SCSI adapters, they should be avoided. After factoring for time lost, frustration, and corrupted data, Adaptec's will prove to be the costliest mistake you'll ever make. That said, if you have SMP problems with 2.1.88, try the patch ftp://ftp.bero-online.ml.org/pub/linux/aic7xxx-5.0.7-linux21.tar.gz I am not sure if this patch has been pulled into later 2.1.x kernels. For further info, take a look at the m