Commit 845abeb1 authored by Hans Holmberg's avatar Hans Holmberg Committed by Carlos Maiolino
Browse files

xfs: add tunable threshold parameter for triggering zone GC 



Presently we start garbage collection late - when we start running
out of free zones to backfill max_open_zones. This is a reasonable
default as it minimizes write amplification. The longer we wait,
the more blocks are invalidated and reclaim cost less in terms
of blocks to relocate.

Starting this late however introduces a risk of GC being outcompeted
by user writes. If GC can't keep up, user writes will be forced to
wait for free zones with high tail latencies as a result.

This is not a problem under normal circumstances, but if fragmentation
is bad and user write pressure is high (multiple full-throttle
writers) we will "bottom out" of free zones.

To mitigate this, introduce a zonegc_low_space tunable that lets the
user specify a percentage of how much of the unused space that GC
should keep available for writing. A high value will reclaim more of
the space occupied by unused blocks, creating a larger buffer against
write bursts.

This comes at a cost as write amplification is increased. To
illustrate this using a sample workload, setting zonegc_low_space to
60% avoids high (500ms) max latencies while increasing write
amplification by 15%.

Signed-off-by: default avatarHans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarCarlos Maiolino <cem@kernel.org>
parent a1a56f54

Documentation/admin-guide/xfs.rst

+21 −0
Original line number Diff line number Diff line
@@ -542,3 +542,24 @@ The interesting knobs for XFS workqueues are as follows: 
  nice           Relative priority of scheduling the threads.  These are the

                 same nice levels that can be applied to userspace processes.

============     ===========



Zoned Filesystems

=================



For zoned file systems, the following attributes are exposed in:



 /sys/fs/xfs/<dev>/zoned/



 max_open_zones                 (Min:  1  Default:  Varies  Max:  UINTMAX)

        This read-only attribute exposes the maximum number of open zones

        available for data placement. The value is determined at mount time and

        is limited by the capabilities of the backing zoned device, file system

        size and the max_open_zones mount option.



 zonegc_low_space               (Min:  0  Default:  0  Max:  100)

        Define a percentage for how much of the unused space that GC should keep

        available for writing. A high value will reclaim more of the space

        occupied by unused blocks, creating a larger buffer against write

        bursts at the cost of increased write amplification.  Regardless

        of this value, garbage collection will always aim to free a minimum

        amount of blocks to keep max_open_zones open for data placement purposes.

fs/xfs/xfs_mount.h

+1 −0
Original line number Diff line number Diff line
@@ -229,6 +229,7 @@ typedef struct xfs_mount { 
	bool			m_finobt_nores; /* no per-AG finobt resv. */

	bool			m_update_sb;	/* sb needs update in mount */

	unsigned int		m_max_open_zones;

	unsigned int		m_zonegc_low_space;



	/*

	 * Bitsets of per-fs metadata that have been checked and/or are sick.

fs/xfs/xfs_sysfs.c

+32 −0
Original line number Diff line number Diff line
@@ -718,8 +718,40 @@ max_open_zones_show( 
}

XFS_SYSFS_ATTR_RO(max_open_zones);



static ssize_t

zonegc_low_space_store(

	struct kobject		*kobj,

	const char		*buf,

	size_t			count)

{

	int			ret;

	unsigned int		val;



	ret = kstrtouint(buf, 0, &val);

	if (ret)

		return ret;



	if (val > 100)

		return -EINVAL;



	zoned_to_mp(kobj)->m_zonegc_low_space = val;



	return count;

}



static ssize_t

zonegc_low_space_show(

	struct kobject		*kobj,

	char			*buf)

{

	return sysfs_emit(buf, "%u\n",

			zoned_to_mp(kobj)->m_zonegc_low_space);

}

XFS_SYSFS_ATTR_RW(zonegc_low_space);



static struct attribute *xfs_zoned_attrs[] = {

	ATTR_LIST(max_open_zones),

	ATTR_LIST(zonegc_low_space),

	NULL,

};

ATTRIBUTE_GROUPS(xfs_zoned);

fs/xfs/xfs_zone_alloc.c

+7 −0
Original line number Diff line number Diff line
@@ -1201,6 +1201,13 @@ xfs_mount_zones( 
	xfs_set_freecounter(mp, XC_FREE_RTEXTENTS,

			iz.available + iz.reclaimable);



	/*

	 * The user may configure GC to free up a percentage of unused blocks.

	 * By default this is 0. GC will always trigger at the minimum level

	 * for keeping max_open_zones available for data placement.

	 */

	mp->m_zonegc_low_space = 0;



	error = xfs_zone_gc_mount(mp);

	if (error)

		goto out_free_zone_info;

fs/xfs/xfs_zone_gc.c

+14 −2
Original line number Diff line number Diff line
@@ -162,18 +162,30 @@ struct xfs_zone_gc_data { 


/*

 * We aim to keep enough zones free in stock to fully use the open zone limit

 * for data placement purposes.

 * for data placement purposes. Additionally, the m_zonegc_low_space tunable

 * can be set to make sure a fraction of the unused blocks are available for

 * writing.

 */

bool

xfs_zoned_need_gc(

	struct xfs_mount	*mp)

{

	s64			available, free;



	if (!xfs_group_marked(mp, XG_TYPE_RTG, XFS_RTG_RECLAIMABLE))

		return false;

	if (xfs_estimate_freecounter(mp, XC_FREE_RTAVAILABLE) <



	available = xfs_estimate_freecounter(mp, XC_FREE_RTAVAILABLE);



	if (available <

	    mp->m_groups[XG_TYPE_RTG].blocks *

	    (mp->m_max_open_zones - XFS_OPEN_GC_ZONES))

		return true;



	free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);

	if (available < mult_frac(free, mp->m_zonegc_low_space, 100))

		return true;



	return false;

}