svn commit: r355837 - head/sys/cam

[Steven Hartland]

Sticky keyboard there Warner?

On a more serious note the fact that the controllers lie about the 
underlying
location of data, the impact of skipping the TRIM requests can have a 
much more
serious impact than one might think depending on the drive, so this type of
optimisation can significantly harm performance instead of increasing it.

This was the main reasons we sponsored the initial ZFS TRIM 
implementation; as
drive performance go so bad with no TRIM that SSD's performed worse than 
HDD's.

Now obviously this was some time ago, but I wouldn't be surprised if 
there's bad
hardware / firmware like this still being produced.

Given that might be a good idea to make this optional, possibly even opt 
in not opt
out?

     Regards
     Steve

On 17/12/2019 00:13, Warner Losh wrote:
> Author: imp
> Date: Tue Dec 17 00:13:45 2019
> New Revision: 355837
> URL: https://svnweb.freebsd.org/changeset/base/355837
>
> Log:
>    Implement bio_speedup
>    
>    React to the BIO_SPEED command in the cam io scheduler by completing
>    as successful BIO_DELETE commands that are pending, up to the length
>    passed down in the BIO_SPEEDUP cmomand. The length passed down is a
>    hint for how much space on the drive needs to be recovered. By
>    completing the BIO_DELETE comomands, this allows the upper layers to
>    allocate and write to the blocks that were about to be trimmed. Since
>    FreeBSD implements TRIMSs as advisory, we can eliminliminate them and
>    go directly to writing.
>    
>    The biggest benefit from TRIMS coomes ffrom the drive being able t
>    ooptimize its free block pool inthe log run. There's little nto no
>    bene3efit in the shoort term. , sepeciall whn the trim is followed by
>    a write. Speedup lets  us make this tradeoff.
>    
>    Reviewed by: kirk, kib
>    Sponsored by: Netflix
>    Differential Revision: https://reviews.freebsd.org/D18351
>
> Modified:
>    head/sys/cam/cam_iosched.c
>
> Modified: head/sys/cam/cam_iosched.c
> ==============================================================================
> --- head/sys/cam/cam_iosched.c	Tue Dec 17 00:13:40 2019	(r355836)
> +++ head/sys/cam/cam_iosched.c	Tue Dec 17 00:13:45 2019	(r355837)
> @@ -1534,6 +1534,41 @@ cam_iosched_queue_work(struct cam_iosched_softc *isc,
>   {
>   
>   	/*
> +	 * A BIO_SPEEDUP from the uppper layers means that they have a block
> +	 * shortage. At the present, this is only sent when we're trying to
> +	 * allocate blocks, but have a shortage before giving up. bio_length is
> +	 * the size of their shortage. We will complete just enough BIO_DELETEs
> +	 * in the queue to satisfy the need. If bio_length is 0, we'll complete
> +	 * them all. This allows the scheduler to delay BIO_DELETEs to improve
> +	 * read/write performance without worrying about the upper layers. When
> +	 * it's possibly a problem, we respond by pretending the BIO_DELETEs
> +	 * just worked. We can't do anything about the BIO_DELETEs in the
> +	 * hardware, though. We have to wait for them to complete.
> +	 */
> +	if (bp->bio_cmd == BIO_SPEEDUP) {
> +		off_t len;
> +		struct bio *nbp;
> +
> +		len = 0;
> +		while (bioq_first(&isc->trim_queue) &&
> +		    (bp->bio_length == 0 || len < bp->bio_length)) {
> +			nbp = bioq_takefirst(&isc->trim_queue);
> +			len += nbp->bio_length;
> +			nbp->bio_error = 0;
> +			biodone(nbp);
> +		}
> +		if (bp->bio_length > 0) {
> +			if (bp->bio_length > len)
> +				bp->bio_resid = bp->bio_length - len;
> +			else
> +				bp->bio_resid = 0;
> +		}
> +		bp->bio_error = 0;
> +		biodone(bp);
> +		return;
> +	}
> +
> +	/*
>   	 * If we get a BIO_FLUSH, and we're doing delayed BIO_DELETEs then we
>   	 * set the last tick time to one less than the current ticks minus the
>   	 * delay to force the BIO_DELETEs to be presented to the client driver.
> @@ -1919,8 +1954,8 @@ DB_SHOW_COMMAND(iosched, cam_iosched_db_show)
>   	db_printf("Trim Q len         %d\n", biolen(&isc->trim_queue));
>   	db_printf("read_bias:         %d\n", isc->read_bias);
>   	db_printf("current_read_bias: %d\n", isc->current_read_bias);
> -	db_printf("Trims active       %d\n", isc->pend_trim);
> -	db_printf("Max trims active   %d\n", isc->max_trim);
> +	db_printf("Trims active       %d\n", isc->pend_trims);
> +	db_printf("Max trims active   %d\n", isc->max_trims);
>   }
>   #endif
>   #endif