Next Previous Contents

## 5. RAID setup

## 5.1 General setup

This is what you need for any of the RAID levels:

- A kernel. Preferably a kernel from the 2.4 series. Alternatively a 2.0 or 2.2 kernel with the RAID patches applied.
- The RAID tools.
- Patience, Pizza, and your favorite caffeinated beverage.

All of this is included as standard in most GNU/Linux distributions today.

If your system has RAID support, you should have a file called `/proc/mdstat`. Remember it, that file is your friend. If you do not have that file, maybe your kernel does not have RAID support. See what the contains, by doing a `cat` `/proc/mdstat`. It should tell you that you have the right RAID personality (eg. RAID mode) registered, and that no RAID devices are currently active.

Create the partitions you want to include in your RAID set.

## 5.2 Downloading and installing the RAID tools

The RAID tools are included in almost every major Linux distribution.

IMPORTANT: If using Debian Woody (3.0) or later, you can install the package by running

    apt-get install raidtools2

This `raidtools2` is a modern version of the old `raidtools` package, which doesn't support the persistent-superblock and parity-algorithm settings.

## 5.3 Downloading and installing mdadm

You can download the most recent mdadm tarball at http://www.cse.unsw.edu.au/~neilb/source/mdadm/. Issue a nice `make install` to compile and then install mdadm and its documentation, manual pages and example files.

    tar xvf ./mdadm-1.4.0.tgz
    cd mdadm-1.4.0.tgz
    make install

If using an RPM-based distribution, you can download and install the package file found at http://www.cse.unsw.edu.au/~neilb/source/mdadm/RPM.

    rpm -ihv mdadm-1.4.0-1.i386.rpm

If using Debian Woody (3.0) or later, you can install the package by running

    apt-get install mdadm

Gentoo has this package available in the portage tree. There you can run

    emerge mdadm

Other distributions may also have this package available. Now, let's go mode-specific.

## 5.4 Linear mode

Ok, so you have two or more partitions which are not necessarily the same size (but of course can be), which you want to append to each other.

Set up the `/etc/raidtab` file to describe your setup. I set up a raidtab for two disks in linear mode, and the file looked like this:

    raiddev /dev/md0
            raid-level      linear
            nr-raid-disks   2
            chunk-size      32
            persistent-superblock 1
            device          /dev/sdb6
            raid-disk       0
            device          /dev/sdc5
            raid-disk       1

Spare-disks are not supported here. If a disk dies, the array dies with it. There's no information to put on a spare disk.

You're probably wondering why we specify a `chunk-size` here when linear mode just appends the disks into one large array with no parallelism. Well, you're completely right, it's odd. Just put in some chunk size and don't worry about this any more.

Ok, let's create the array. Run the command

      mkraid /dev/md0

This will initialize your array, write the persistent superblocks, and start the array.

If you are using mdadm, a single command like

       mdadm --create --verbose /dev/md0 --level=linear --raid-devices=2 /dev/sdb6 /dev/sdc5

should create the array. The parameters talk for themselves. The output might look like this

      mdadm: chunk size defaults to 64K
      mdadm: array /dev/md0 started.

Have a look in `/proc/mdstat`. You should see that the array is running.

Now, you can create a filesystem, just like you would on any other device, mount it, include it in your `/etc/fstab` and so on.

## 5.5 RAID-0

You have two or more devices, of approximately the same size, and you want to combine their storage capacity and also combine their performance by accessing them in parallel.

Set up the `/etc/raidtab` file to describe your configuration. An example raidtab looks like:

    raiddev /dev/md0
            raid-level      0
            nr-raid-disks   2
            persistent-superblock 1
            chunk-size     4
            device          /dev/sdb6
            raid-disk       0
            device          /dev/sdc5
            raid-disk       1

Like in Linear mode, spare disks are not supported here either. RAID-0 has no redundancy, so when a disk dies, the array goes with it.

Again, you just run

      mkraid /dev/md0

to initialize the array. This should initialize the superblocks and start the raid device. Have a look in `/proc/mdstat` to see what's going on. You should see that your device is now running.

/dev/md0 is now ready to be formatted, mounted, used and abused.

## 5.6 RAID-1

You have two devices of approximately same size, and you want the two to be mirrors of each other. Eventually you have more devices, which you want to keep as stand-by spare-disks, that will automatically become a part of the mirror if one of the active devices break.

Set up the `/etc/raidtab` file like this:

    raiddev /dev/md0
            raid-level      1
            nr-raid-disks   2
            nr-spare-disks  0
            persistent-superblock 1
            device          /dev/sdb6
            raid-disk       0
            device          /dev/sdc5
            raid-disk       1

If you have spare disks, you can add them to the end of the device specification like

            device          /dev/sdd5
            spare-disk      0

Remember to set the `nr-spare-disks` entry correspondingly.

Ok, now we're all set to start initializing the RAID. The mirror must be constructed, eg. the contents (however unimportant now, since the device is still not formatted) of the two devices must be synchronized.

Issue the

      mkraid /dev/md0

command to begin the mirror initialization.

Check out the `/proc/mdstat` file. It should tell you that the /dev/md0 device has been started, that the mirror is being reconstructed, and an ETA of the completion of the reconstruction.

Reconstruction is done using idle I/O bandwidth. So, your system should still be fairly responsive, although your disk LEDs should be glowing nicely.

The reconstruction process is transparent, so you can actually use the device even though the mirror is currently under reconstruction.

Try formatting the device, while the reconstruction is running. It will work. Also you can mount it and use it while reconstruction is running. Of Course, if the wrong disk breaks while the reconstruction is running, you're out of luck.

## 5.7 RAID-4

Note! I haven't tested this setup myself. The setup below is my best guess, not something I have actually had up running. If you use RAID-4, please write to the author and share your experiences.

You have three or more devices of roughly the same size, one device is significantly faster than the other devices, and you want to combine them all into one larger device, still maintaining some redundancy information. Eventually you have a number of devices you wish to use as spare-disks.

Set up the `/etc/raidtab` file like this:

    raiddev /dev/md0
            raid-level      4
            nr-raid-disks   4
            nr-spare-disks  0
            persistent-superblock 1
            chunk-size      32
            device          /dev/sdb1
            raid-disk       0
            device          /dev/sdc1
            raid-disk       1
            device          /dev/sdd1
            raid-disk       2
            device          /dev/sde1
            raid-disk       3

If we had any spare disks, they would be inserted in a similar way, following the raid-disk specifications;

            device         /dev/sdf1
            spare-disk     0

as usual.

Your array can be initialized with the

       mkraid /dev/md0

command as usual.

You should see the section on special options for mke2fs before formatting the device.

## 5.8 RAID-5

You have three or more devices of roughly the same size, you want to combine them into a larger device, but still to maintain a degree of redundancy for data safety. Eventually you have a number of devices to use as spare-disks, that will not take part in the array before another device fails.

If you use N devices where the smallest has size S, the size of the entire array will be (N-1)*S. This "missing" space is used for parity (redundancy) information. Thus, if any disk fails, all data stay intact. But if two disks fail, all data is lost.

Set up the `/etc/raidtab` file like this:

    raiddev /dev/md0
            raid-level      5
            nr-raid-disks   7
            nr-spare-disks  0
            persistent-superblock 1
            parity-algorithm        left-symmetric
            chunk-size      32
            device          /dev/sda3
            raid-disk       0
            device          /dev/sdb1
            raid-disk       1
            device          /dev/sdc1
            raid-disk       2
            device          /dev/sdd1
            raid-disk       3
            device          /dev/sde1
            raid-disk       4
            device          /dev/sdf1
            raid-disk       5
            device          /dev/sdg1
            raid-disk       6

If we had any spare disks, they would be inserted in a similar way, following the raid-disk specifications;

            device         /dev/sdh1
            spare-disk     0

And so on.

A chunk size of 32 kB is a good default for many general purpose filesystems of this size. The array on which the above raidtab is used, is a 7 times 6 GB = 36 GB (remember the (n-1)*s = (7-1)*6 = 36) device. It holds an ext2 filesystem with a 4 kB block size. You could go higher with both array chunk-size and filesystem block-size if your filesystem is either much larger, or just holds very large files.

Ok, enough talking. You set up the `/etc/raidtab`, so let's see if it works. Run the

      mkraid /dev/md0

command, and see what happens. Hopefully your disks start working like mad, as they begin the reconstruction of your array. Have a look in `/proc/mdstat` to see what's going on.

If the device was successfully created, the reconstruction process has now begun. Your array is not consistent until this reconstruction phase has completed. However, the array is fully functional (except for the handling of device failures of course), and you can format it and use it even while it is reconstructing.

See the section on special options for mke2fs before formatting the array.

Ok, now when you have your RAID device running, you can always stop it or re-start it using the

      raidstop /dev/md0

or

      raidstart /dev/md0

commands.

With mdadm you can stop the device using

      mdadm -S /dev/md0

and re-start it with

      mdadm -R /dev/md0

Instead of putting these into init-files and rebooting a zillion times to make that work, read on, and get autodetection running.

## 5.9 The Persistent Superblock

Back in "The Good Old Days" (TM), the raidtools would read your `/etc/raidtab` file, and then initialize the array. However, this would require that the filesystem on which `/etc/raidtab` resided was mounted. This is unfortunate if you want to boot on a RAID.

Also, the old approach led to complications when mounting filesystems on RAID devices. They could not be put in the `/etc/fstab` file as usual, but would have to be mounted from the init-scripts.

The persistent superblocks solve these problems. When an array is initialized with the `persistent-superblock` option in the `/etc/raidtab` file, a special superblock is written in the beginning of all disks participating in the array. This allows the kernel to read the configuration of RAID devices directly from the disks involved, instead of reading from some configuration file that may not be available at all times.

You should however still maintain a consistent `/etc/raidtab` file, since you may need this file for later reconstruction of the array.

The persistent superblock is mandatory if you want auto-detection of your RAID devices upon system boot. This is described in the Autodetection section.

## 5.10 Chunk sizes

The chunk-size deserves an explanation. You can never write completely parallel to a set of disks. If you had two disks and wanted to write a byte, you would have to write four bits on each disk, actually, every second bit would go to disk 0 and the others to disk 1. Hardware just doesn't support that. Instead, we choose some chunk-size, which we define as the smallest "atomic" mass of data that can be written to the devices. A write of 16 kB with a chunk size of 4 kB, will cause the first and the third 4 kB chunks to be written to the first disk, and the second and fourth chunks to be written to the second disk, in the RAID-0 case with two disks. Thus, for large writes, you may see lower overhead by having fairly large chunks, whereas arrays that are primarily holding small files may benefit more from a smaller chunk size.

Chunk sizes must be specified for all RAID levels, including linear mode. However, the chunk-size does not make any difference for linear mode.

For optimal performance, you should experiment with the value, as well as with the block-size of the filesystem you put on the array.

The argument to the chunk-size option in `/etc/raidtab` specifies the chunk-size in kilobytes. So "4" means "4 kB".

### RAID-0

Data is written "almost" in parallel to the disks in the array. Actually, `chunk-size` bytes are written to each disk, serially.

If you specify a 4 kB chunk size, and write 16 kB to an array of three disks, the RAID system will write 4 kB to disks 0, 1 and 2, in parallel, then the remaining 4 kB to disk 0.

A 32 kB chunk-size is a reasonable starting point for most arrays. But the optimal value depends very much on the number of drives involved, the content of the file system you put on it, and many other factors. Experiment with it, to get the best performance.

### RAID-0 with ext2

The following tip was contributed by michael@freenet-ag.de:

There is more disk activity at the beginning of ext2fs block groups. On a single disk, that does not matter, but it can hurt RAID0, if all block groups happen to begin on the same disk. Example:

With 4k stripe size and 4k block size, each block occupies one stripe. With two disks, the stripe-#disk-product is 2*4k=8k. The default block group size is 32768 blocks, so all block groups start on disk 0, which can easily become a hot spot, thus reducing overall performance. Unfortunately, the block group size can only be set in steps of 8 blocks (32k when using 4k blocks), so you can not avoid the problem by adjusting the block group size with the -g option of mkfs(8).

If you add a disk, the stripe-#disk-product is 12, so the first block group starts on disk 0, the second block group starts on disk 2 and the third on disk 1. The load caused by disk activity at the block group beginnings spreads over all disks.

In case you can not add a disk, try a stripe size of 32k. The stripe-#disk-product is 64k. Since you can change the block group size in steps of 8 blocks (32k), using a block group size of 32760 solves the problem.

Additionally, the block group boundaries should fall on stripe boundaries. That is no problem in the examples above, but it could easily happen with larger stripe sizes.

### RAID-1

For writes, the chunk-size doesn't affect the array, since all data must be written to all disks no matter what. For reads however, the chunk-size specifies how much data to read serially from the participating disks. Since all active disks in the array contain the same information, the RAID layer has complete freedom in choosing from which disk information is read - this is used by the RAID code to improve average seek times by picking the disk best suited for any given read operation.

### RAID-4

When a write is done on a RAID-4 array, the parity information must be updated on the parity disk as well.

The chunk-size affects read performance in the same way as in RAID-0, since reads from RAID-4 are done in the same way.

### RAID-5

On RAID-5, the chunk size has the same meaning for reads as for RAID-0. Writing on RAID-5 is a little more complicated: When a chunk is written on a RAID-5 array, the corresponding parity chunk must be updated as well. Updating a parity chunk requires either

- The original chunk, the new chunk, and the old parity block
- Or, all chunks (except for the parity chunk) in the stripe

The RAID code will pick the easiest way to update each parity chunk as the write progresses. Naturally, if your server has lots of memory and/or if the writes are nice and linear, updating the parity chunks will only impose the overhead of one extra write going over the bus (just like RAID-1). The parity calculation itself is extremely efficient, so while it does of course load the main CPU of the system, this impact is negligible. If the writes are small and scattered all over the array, the RAID layer will almost always need to read in all the untouched chunks from each stripe that is written to, in order to calculate the parity chunk. This will impose extra bus-overhead and latency due to extra reads.

A reasonable chunk-size for RAID-5 is 128 kB, but as always, you may want to experiment with this.

Also see the section on special options for mke2fs. This affects RAID-5 performance.

## 5.11 Options for mke2fs

There is a special option available when formatting RAID-4 or -5 devices with mke2fs. The `-R stride=nn` option will allow mke2fs to better place different ext2 specific data-structures in an intelligent way on the RAID device.

If the chunk-size is 32 kB, it means, that 32 kB of consecutive data will reside on one disk. If we want to build an ext2 filesystem with 4 kB block-size, we realize that there will be eight filesystem blocks in one array chunk. We can pass this information on the mke2fs utility, when creating the filesystem:

      mke2fs -b 4096 -R stride=8 /dev/md0

RAID-{4,5} performance is severely influenced by this option. I am unsure how the stride option will affect other RAID levels. If anyone has information on this, please send it in my direction.

The ext2fs blocksize severely influences the performance of the filesystem. You should always use 4kB block size on any filesystem larger than a few hundred megabytes, unless you store a very large number of very small files on it.

Next Previous Contents