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When running "lfs mirror extend" to add a mirror to an existing file, it appears that the OST(s) used for the original layout are not taken into account, if OST pools or indices are not explicitly specified:
in this case, both the original and mirror copy of the file are allocated on OST0001 because it has the most free space, which isn't helpful in terms of availability or performance for this file.
A previous patch https://review.whamcloud.com/32404 "LU-9007 lod: improve obj alloc for FLR file" added lod_should_avoid_ost() to handle new OST object allocations for different components of a single mirror, but does not appear to handle allocations across FLR mirrors.
Requirements for OST selection on components with overlapping extents should be, in order of decreasing priority:
- objects with overlapping components must not share the same OST. Implies mirror_count <= ost_count / component_stripe_count. In theory this could be relaxed if all replicas have the same stripe_count and stripe_size, then it would only require that the same OST cannot be at the same stripe_index of different components, in which case max replica count == OST count, but this is more difficult to control.
- objects with overlapping components should not share OSTs on the same OSS node (by NID from imp->imp_connection->c_peer.nid, as qos_add_tgt() does) to avoid the shared node failure domain.
- objects with overlapping components should not share OSTs on the same OSS failover pair (by failover NID from imp->imp_conn_list.oic_conn->c_peer.nid, as lprocfs_import_seq_show() does) to avoid the shared storage enclosure/controller failure domain. There may be other OSS nodes that share the same storage enclosure/controller, but there isn't any way for the client to determine this automatically.
- objects with overlapping components should not be on OSTs on the same network switch, power supply, rack, etc. but this depends on external information that is not currently available to Lustre. That could optionally be added via a separate configuration file/options, but the above cases will automatically cover the most important failure scenarios
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