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f8c0d7e1f6
`__vdev_disk_physio()` uses `abd_nr_pages_off()` to allocate a bio with a sufficient number of iovec's to process this zio (i.e. `nr_iovecs`/`bi_max_vecs`). If there are not enough iovec's in the bio, then additional bio's will be allocated. However, this is a sub-optimal code path. In particular, it requires several abd calls (to `abd_nr_pages_off()` and `abd_bio_map_off()`) which will have to walk the constituents of the ABD (the pages or the gang children) because they are looking for offsets > 0. For gang ABD's, `abd_nr_pages_off()` returns the number of iovec's needed for the first constituent, rather than the sum of all constituents (within the requested range). This always under-estimates the required number of iovec's, which causes us to always need several bio's. The end result is that `__vdev_disk_physio()` is usually O(n^2) for gang ABD's (and occasionally O(n^3), when more than 16 bio's are needed). This commit fixes `abd_nr_pages_off()`'s handling of gang ABD's, to correctly determine how many iovec's are needed, by adding up the number of iovec's for each of the gang children in the requested range. Reviewed-by: Mark Maybee <mark.maybee@delphix.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Brian Atkinson <batkinson@lanl.gov> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #11536 |
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