diff options
| author | Richard Yao <[email protected]> | 2014-07-11 14:35:58 -0400 |
|---|---|---|
| committer | Brian Behlendorf <[email protected]> | 2017-05-04 18:00:27 -0400 |
| commit | bc17f1047a83cc8c4065e0ef84333a0d9b9d73aa (patch) | |
| tree | 240277a2f851d5bcfe4839dcb7e720dd58662945 /module | |
| parent | 5731140eaf4aaf2526a8bfdbfe250195842e79eb (diff) | |
Enable Linux read-ahead for a single page on ZVOLs
Linux has read-ahead logic designed to accelerate sequential workloads.
ZFS has its own read-ahead logic called zprefetch that operates on both
ZVOLs and datasets. Having two prefetchers active at the same time can
cause overprefetching, which unnecessarily reduces IOPS performance on
CoW filesystems like ZFS.
Testing shows that entirely disabling the Linux prefetch results in
a significant performance penalty for reads while commensurate benefits
are seen in random writes. It appears that read-ahead benefits are
inversely proportional to random write benefits, and so a single page
of Linux-layer read-ahead appears to offer the middle ground for both
workloads.
Reviewed-by: Chunwei Chen <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Richard Yao <[email protected]>
Issue #5902
Diffstat (limited to 'module')
| -rw-r--r-- | module/zfs/zvol.c | 3 |
1 files changed, 3 insertions, 0 deletions
diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c index d0f7b9912..fef1d299a 100644 --- a/module/zfs/zvol.c +++ b/module/zfs/zvol.c @@ -1468,6 +1468,9 @@ zvol_alloc(dev_t dev, const char *name) blk_queue_make_request(zv->zv_queue, zvol_request); blk_queue_set_write_cache(zv->zv_queue, B_TRUE, B_TRUE); + /* Limit read-ahead to a single page to prevent over-prefetching. */ + blk_queue_set_read_ahead(zv->zv_queue, 1); + /* Disable write merging in favor of the ZIO pipeline. */ queue_flag_set(QUEUE_FLAG_NOMERGES, zv->zv_queue); |