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bc97daa07e
Reviewed by: Bayard Bell <buffer.g.overflow@gmail.com> Reviewed by: Alek Pinchuk <alek@nexenta.com> Reviewed by: Simon Klinkert <simon.klinkert@gmail.com> Reviewed by: Dan McDonald <danmcd@omniti.com> Approved by: Richard Lowe <richlowe@richlowe.net> Author: Matthew Ahrens <mahrens@delphix.com> illumos/illumos-gate@46e1baa6cf https://www.illumos.org/issues/5911 Sometimes ZFS appears to hang while deleting a file. It is actually making slow progress at the file deletion, but other operations (administrative and writes via the data path) "hang" until the file removal completes, which can take a long time if the file has many blocks. The deletion (or most of it) happens in a single txg, and the sync thread spends most of its time reading indirect blocks via this stack trace: swtch+0x141() cv_wait+0x70() zio_wait+0x5b() dbuf_read+0x2c0() free_children+0x50() free_children+0x12a() free_children+0x12a() free_children+0x12a() dnode_sync_free_range_impl+0xdf() dnode_sync_free_range+0x52() range_tree_vacate+0x65() dnode_sync+0x1d8() dmu_objset_sync_dnodes+0x77() dmu_objset_sync+0x19f() dsl_dataset_sync+0x51() dsl_pool_sync+0x9a() spa_sync+0x2ff() txg_sync_thread+0x21f() thread_start+8() One way to reproduce the problem is if we are over the arc_meta_limit, e.g. because lots of indirect blocks are pinned because we have L0 dbufs under them. It could be that most of the L1 indirects are cached, in which case when dmu_free_long_range_impl() calls dmu_tx_hold_free(), it will complete very quickly. This allows dmu_free_long_range_impl() to put many (perhaps all of its) transactions in the same TXG. However, dmu_free_long_range_impl() calls dnode_evict_dbufs (and dnode_free_range()), which removes the L0 dbufs, thus reducing the hold count on the L1 indirect blocks above it, allowing them to be evicted. Because we are over the arc_meta_limit(), these L1 blocks will be evicted ASAP. Thus when we get to syncing context, the L1 indirects are no longer cached and must be read in. Obtained from: illumos MFC after: 15 days |
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