Lustre 1.6 Operations Manual

25.1.1 Advantages of Striping
There are two reasons to create files of multiple stripes: bandwidth and size.
25.1.1.1 Bandwidth
There are many applications which require high-bandwidth access to a single file –
more bandwidth than can be provided by a single OSS. For example, scientific
applications which write to a single file from hundreds of nodes or a binary
executable which is loaded by many nodes when an application starts.
In cases like these, stripe your file over as many OSSs as it takes to achieve the
required peak aggregate bandwidth for that file. In our experience, the requirement
is “as quickly as possible,” which usually means all OSSs.
Note – This assumes that your application is using enough client nodes, and can
read/write data fast enough to take advantage of this much OSS bandwidth. The
largest useful stripe count is bounded by the I/O rate of your clients/jobs divided
by the performance per OSS.
25.1.1.2 Size
The second reason to stripe is when a single OST does not have enough free space to
hold the entire file.
There is never an exact, one-to-one mapping between clients and OSTs. Lustre uses a
round-robin algorithm for OST stripe selection until free space on OSTs differ by
more than 20%. However, depending on actual file sizes, some stripes may be mostly
empty, while others are more full. For a more detailed description of stripe
assignments, see Free Space Management.
After every ostcount+1 objects, Lustre skips an OST. This causes Lustre’s "starting
point" to precess around, eliminating some degenerated cases where applications
that create very regular file layouts (striping patterns) would have preferentially
used a particular OST in the sequence.
25-2 Lustre 1.6 Operations Manual • September 2008