KenRawlings: initial page creation as part of accelerated wiki migration

2016-10-13T14:22:31Z

initial page creation as part of accelerated wiki migration

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!Note: This page originated on the old Lustre wiki. It was identified as likely having value and was migrated to the new wiki. It is in the process of being reviewed/updated and may currently have content that is out of date.
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<small>''(Updated: Oct 2009)''</small>
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In Lustre 1.6 and later, the MDT assigns file stripes to OSTs based on location (which
OSS) and size considerations (free space) to optimize file system performance.
Emptier OSTs are preferentially selected for stripes, and stripes are preferentially
spread out between OSSs to increase network bandwidth utilization. The weighting
factor between these two optimizations can be adjusted by the user.

Two stripe allocation methods are provided: round-robin and weighted. By default, the allocation method is determined by the amount of free-space imbalance on the OSTs. The
weighted allocator is used when any two OSTs are imbalanced by more than 20%.
Otherwise, the faster round-robin allocator is used. (The round-robin order maximizes
network balancing.)

=== Round-Robin Allocator ===
When OSTs have approximately the same amount of free space (within 20%), an
efficient round-robin allocator is used. The round-robin allocator alternates stripes
between OSTs on different OSSs, so the OST used for stripe 0 of each file is evenly distributed among OSTs, regardless of the stripe count. Shown below are several sample round-robin stripe orders
(each letter represents a different OST on a single OSS):
: 3: AAA (one 3-OST OSS)
: 3x3: ABABAB (two 3-OST OSSs)
: 3x4: BBABABA (one 3-OST OSS (A) and one 4-OST OSS (B))
: 3x5: BBABBABA
: 3x5x1: BBABABABC
: 3x5x2: BABABCBABC
: 4x6x2: BABABCBABABC

=== Weighted Allocator ===
When the free space difference between the OSTs is significant, then a weighting
algorithm is used to influence OST ordering based on size and location. Note that
these are weightings for a random algorithm, so the OST with the most free space is not
necessarily chosen every time. On average, the weighted allocator fills the emptier
OSTs faster.

=== Adjusting the Weighting Between Free Space and Location ===
The weighting priority can be adjusted in the ''proc'' file ''/proc/fs/lustre/lov/lustremdtlov/qos_prio_free''. The default value is 90%.

Use the following command on the MGS to change this weighting:

lctl conf_param <fsname>-MDT0000.lov.qos_prio_free=90

Increasing the value puts more weighting on free space. When the free space priority
is set to 100%, then location is no longer used in stripe-ordering calculations and
weighting is based entirely on free space.

'''''Note:''''' Setting the priority to 100% means that OSS distribution does not count in the weighting, but the stripe assignment is still done via a weighting. For example, if OST2 has twice as much free space as OST1, then OST2 is twice as likely to be used, but it is not guaranteed to be used.

[[Category: NeedsReview]]

Managing Free Space - Revision history

KenRawlings: initial page creation as part of accelerated wiki migration