Block Device Performance Tuning

1. Tools

Kprobe, used to catch some real-time data

Iostate-kx 1/dev/*

Blktrace-d/dev/dm-33-o-|blkparse-i-o blkparse.out

Http://blog.chinaunix.net/uid-24774106-id-4096470.html

2. Through trace discovery, sometimes the bio out of the queue is done by the KBLOCKD process, and kblockd every 3ms execution (blk_unplug_timeout), which is the reason for poor performance.

But sometimes, it is in the current context out of the queue, not through the IO scheduling subsystem (should be called __generic_unplug_device).

3. Blk_queue_make_request (), this function inside set q->unplug_delay = Msecs_to_jiffies (3), 3ms wake Kblockd once.

　　

Block devices under the device mapped framework:

Dm_request () "Bio's entrance"

--Blk_queue_bio (q, bio)

-Blk_queue_bio

In the Blk_queue_bio function, if bio sets the Bio_rw_unplug bit, the const bool unplug = bio_rw_flagged (bio, bio_rw_unplug);

The __generic_unplug_device () is called, that is, the bio can be forwarded in the current context.

Block Device Performance Tuning