[Lustre-discuss] OST size limitation

[Kevin Van Maren]

On Nov 2, 2011, at 1:48 PM, Charland, Denis wrote:

I read in the Lustre Operations Manual that there is an OST size limitation of 16 TB on RHEL and
8 TB on other distributions because of the ext3 file system limitation. I have a few questions about that.

Why is the limitation 16 TB on RHEL?


16TB is the maximum size RedHat supports.  See http://www.redhat.com/rhel/compare/
Larger than that requires bigger changes.

Note that whamcloud's 1.8.6-wc1 claimed support for 24TB LUNs (but see http://jira.whamcloud.com/browse/LU-419 ).

Whamcloud's Lustre 2.1 (not sure you'd want to use it) claims support for 128TB LUNs.



I plan to use Lustre 1.8.5 on Fedora 12 for a new Lustre file system. What will be the OST size limitation?

What is the OST size limitation when using ext4?

16TB with the Lustre-patched RHEL kernel.


Is it preferable to use ext4 instead of ext3?

If the block device has more than 8 TB or 16 TB, it must be partitioned. Is there a performance degradation
when a device has multiple partitions compared to a single partition? In other words, is it better to have three
8 TB devices with one partition per device than to have one 24 TB device with three partitions?


Better to have 3 separate 8TB LUNs.  Different OSTs forcing the same drive heads to move to opposite parts of the disk does degrade performance (with a single OST moving the drive heads, the block allocator tries to minimize movement).



Denis Charland
UNIX Systems Administrator
National Research Council Canada

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