summaryrefslogtreecommitdiffstats
path: root/module/zfs/dmu_traverse.c
diff options
context:
space:
mode:
authorMatthew Ahrens <[email protected]>2020-03-10 10:51:04 -0700
committerGitHub <[email protected]>2020-03-10 10:51:04 -0700
commit1dc32a67e93bbc8d650943f1a460abb9ff6c5083 (patch)
treeba8ab8eb3c5a7beeaabb62c85456c2e0f38d2ee0 /module/zfs/dmu_traverse.c
parent9be70c37844c5d832ffef6f0ed2651a332b35dbd (diff)
Improve zfs send performance by bypassing the ARC
When doing a zfs send on a dataset with small recordsize (e.g. 8K), performance is dominated by the per-block overheads. This is especially true with `zfs send --compressed`, which further reduces the amount of data sent, for the same number of blocks. Several threads are involved, but the limiting factor is the `send_prefetch` thread, which is 100% on CPU. The main job of the `send_prefetch` thread is to issue zio's for the data that will be needed by the main thread. It does this by calling `arc_read(ARC_FLAG_PREFETCH)`. This has an immediate cost of creating an arc_hdr, which takes around 14% of one CPU. It also induces later costs by other threads: * Since the data was only prefetched, dmu_send()->dmu_dump_write() will need to call arc_read() again to get the data. This will have to look up the arc_hdr in the hash table and copy the data from the scatter ABD in the arc_hdr to a linear ABD in arc_buf. This takes 27% of one CPU. * dmu_dump_write() needs to arc_buf_destroy() This takes 11% of one CPU. * arc_adjust() will need to evict this arc_hdr, taking about 50% of one CPU. All of these costs can be avoided by bypassing the ARC if the data is not already cached. This commit changes `zfs send` to check for the data in the ARC, and if it is not found then we directly call `zio_read()`, reading the data into a linear ABD which is used by dmu_dump_write() directly. The performance improvement is best expressed in terms of how many blocks can be processed by `zfs send` in one second. This change increases the metric by 50%, from ~100,000 to ~150,000. When the amount of data per block is small (e.g. 2KB), there is a corresponding reduction in the elapsed time of `zfs send >/dev/null` (from 86 minutes to 58 minutes in this test case). In addition to improving the performance of `zfs send`, this change makes `zfs send` not pollute the ARC cache. In most cases the data will not be reused, so this allows us to keep caching useful data in the MRU (hit-once) part of the ARC. Reviewed-by: Paul Dagnelie <[email protected]> Reviewed-by: Serapheim Dimitropoulos <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Matthew Ahrens <[email protected]> Closes #10067
Diffstat (limited to 'module/zfs/dmu_traverse.c')
0 files changed, 0 insertions, 0 deletions