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Limits of RAID cards, file systems, and OS's
Posted on 08-02-2005 15:22:00 UTC | Updated on 06-03-2008 17:25:47 UTC
Section: /software/linux/ | Permanent Link

I've been doing research on new RAID solutions that can use over 2TB of disk space. I have come upon some interesting info on the limits of the different parts of the system. It seems that there is a big line you have to follow when you want to address large volumes of disk space. Each part of the chain has a limit. Which means the smallest part of the chain is what you are limited to. The chain goes like this Raid card-> OS Kernel-> File system -> Partitioning program.

First the RAID card has to support disks (arrays) or logical volumes larger than 2 terabytes (TB). This is usually done by having the card support 64 bit Logical Block Addressing (LBA). Most cards and OS's unless they are very new only support 32 bit LBA's. This is what limits them to 2TB's (2^32 * 512 ~ 2TB).

Next, the OS needs to be able to format and address a logical volume or disk greater than 2TB. Right now some OS's that do this are Windows 2003 Server SP1, XP 64bit, and the 2.6.x linux kernel. The logical volume can be split into partitions but my example here is using the logical volume being used as one big partition.

Then, you will need a file system that can support a disk/volume over 2TB. The thing with file systems is that they can theoretically get to a certain size but usually there seems to be a limit with another part of the OS that keeps them from reaching this size. Like a driver (NTFS) or a kernel block size limit (linux). FAT16 partitions are limited to a maximum of 2Gb. FAT32's limit is theoretically 8TB but Windows 2000 and Windows XP cannot FORMAT a volume larger than 32 GB in size using the native FAT32 file system. A quick note on FAT32. The file size limit is 4Gig so copying that full dual layer DVD won't work. So to get around all this you'll want to use NTFS. Using the default cluster size of 4 KB for large volumes, the maximum NTFS volume size windows allows is 256 terabytes. This is limited by the windows driver.

In the 2.6 linux kernel the maximum limit of block devices with a 32-bit CPU and kernel is 16TB with 4 KB block sizes. The linux kernel page cache index limit (32 bits) == filesystem block number. 2^32 * 4 KB = 16 TiB. With a 64-bit CPU and 64-bit 2.6 kernel the max block device size is 8 Exabytes (EB). Some filesystems can go to the Exabyte range others can't. For example the Ext3 filesystem on linux can only support a maximum volume size up to 32TB on 64-bit systems. That means you can have a partition max of 32TB. With the XFS filesystem on linux the (partitions) limit is the kernel's block device limit of 16TB on 32-bit and 8EB on 64-bit. The XFS filesystem itself can handle an maximum file size of 8EB and can handle a maximum volume size of 8EB. See the Wikipedia page on comparison of filesystems for more info on the limits of many filesystems. Just remember that just because the filesystem says it can handle something does not mean the kernel or software of the os your running can handle it.

Lastly, you will need a partitioning program that can address as disk/volume over 2TB. For linux as of this writing fdisk can not do this. You would need to use a linux program called parted. With the windows OS's that support the larger LBA's I assume the disk manager can deal with theses new sizes. Just note that Disk devices with more than 2 TB of disk space must be converted to GPT (GUID Partition Tables) format for all of the disk space to be usable. If the device uses MBR format, the disk space beyond 2 TB will be unusable.

The following is a example of parted commands to make raid volume that is accessed as /dev/sda. It uses XFS a the file system.:

parted /dev/sda

Now you should be at a Parted prompt. Next we make the GPT label.

mklabel gpt

Then we see how big our volume is by printing info about it.

p

Disk geometry for /dev/sda: 0.000-2384185.000 megabytes
Disk label type: gpt
Minor       Start       End       Filesystem Name       Flags

Now we make our partition using the number we got from our print out of the volume.

mkpart
Partition type? [primary]? [press enter]
File system type? [ext2]? xfs
Start? 0
End? 2384185

Then we quit.

q

Next make your file system (Ex. mkfs.xfs -f -b size=4k -l size=64m /dev/sda). Don't forget to update /etc/fstab, if necessary.

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