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Subject[PATCH] bcache: make stripe_size configurable and persistent for hardware raid5/6
Date
From: Eric Wheeler <git@linux.ewheeler.net>

While some drivers set queue_limits.io_opt (e.g., md raid5), there are
currently no SCSI/RAID controller drivers that do. Previously stripe_size
and partial_stripes_expensive were read-only values and could not be
tuned by users (eg, for hardware RAID5/6).

This patch enables users to save the optimal IO size via sysfs through
the backing device attributes stripe_size and partial_stripes_expensive
into the bcache superblock.

Superblock changes are backwards-compatable:

* partial_stripes_expensive: One bit was used in the superblock flags field

* stripe_size: There are eight 64-bit "pad" fields for future use in
the superblock which default to 0; from those, 32-bits are now used
to save the stripe_size and load at device registration time.

Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net>
---
Documentation/admin-guide/bcache.rst | 21 +++++++++++++++++++++
drivers/md/bcache/super.c | 15 ++++++++++++++-
drivers/md/bcache/sysfs.c | 33 +++++++++++++++++++++++++++++++--
include/uapi/linux/bcache.h | 6 ++++--
4 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/bcache.rst b/Documentation/admin-guide/bcache.rst
index c0ce64d..ef82022 100644
--- a/Documentation/admin-guide/bcache.rst
+++ b/Documentation/admin-guide/bcache.rst
@@ -420,6 +420,12 @@ dirty_data
label
Name of underlying device.

+partial_stripes_expensive
+ Flag to bcache that partial or unaligned stripe_size'd
+ writes to the backing device are expensive (e.g., RAID5/6 incur
+ read-copy-write). Writing this sysfs attribute updates the superblock
+ and also takes effect immediately. See also stripe_size, below.
+
readahead
Size of readahead that should be performed. Defaults to 0. If set to e.g.
1M, it will round cache miss reads up to that size, but without overlapping
@@ -458,6 +464,21 @@ stop
Write to this file to shut down the bcache device and close the backing
device.

+stripe_size
+ The stripe size in bytes of the backing device for optimial
+ write performance (also known as the "stride width"). This is set
+ automatically when using a device driver sets blk_limits_io_opt
+ (e.g., md, rbd, skd, zram, virtio_blk). No hardware RAID controller
+ sets blk_limits_io_opt as of 2019-06-15, so configure this to suit
+ your needs. Note that you must unregister and re-register the backing
+ device after making a change to stripe_size.
+
+ Where N is the number of data disks,
+ RAID5: stripe_size = (N-1)*RAID_CHUNK_SIZE.
+ RAID6: stripe_size = (N-2)*RAID_CHUNK_SIZE.
+
+ See also partial_stripes_expensive, above.
+
writeback_delay
When dirty data is written to the cache and it previously did not contain
any, waits some number of seconds before initiating writeback. Defaults to
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1b63ac8..d0b9501 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -80,6 +80,7 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,

sb->flags = le64_to_cpu(s->flags);
sb->seq = le64_to_cpu(s->seq);
+ sb->stripe_size = le32_to_cpu(s->stripe_size);
sb->last_mount = le32_to_cpu(s->last_mount);
sb->first_bucket = le16_to_cpu(s->first_bucket);
sb->keys = le16_to_cpu(s->keys);
@@ -221,6 +222,7 @@ static void __write_super(struct cache_sb *sb, struct bio *bio)

out->flags = cpu_to_le64(sb->flags);
out->seq = cpu_to_le64(sb->seq);
+ out->stripe_size = cpu_to_le32(sb->stripe_size);

out->last_mount = cpu_to_le32(sb->last_mount);
out->first_bucket = cpu_to_le16(sb->first_bucket);
@@ -1258,7 +1260,18 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)

dc->disk.stripe_size = q->limits.io_opt >> 9;

- if (dc->disk.stripe_size)
+ if (dc->sb.stripe_size) {
+ if (dc->disk.stripe_size &&
+ dc->disk.stripe_size != dc->sb.stripe_size) {
+ pr_warn("superblock stripe_size (%d) overrides bdev stripe_size (%d)\n",
+ (int)dc->sb.stripe_size,
+ (int)dc->disk.stripe_size);
+ }
+
+ dc->disk.stripe_size = dc->sb.stripe_size;
+ dc->partial_stripes_expensive =
+ (unsigned int)BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb);
+ } else if (dc->disk.stripe_size)
dc->partial_stripes_expensive =
q->limits.raid_partial_stripes_expensive;

diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index bfb437f..4ebca52 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -111,8 +111,8 @@
rw_attribute(writeback_rate_minimum);
read_attribute(writeback_rate_debug);

-read_attribute(stripe_size);
-read_attribute(partial_stripes_expensive);
+rw_attribute(stripe_size);
+rw_attribute(partial_stripes_expensive);

rw_attribute(synchronous);
rw_attribute(journal_delay_ms);
@@ -343,6 +343,35 @@ static ssize_t bch_snprint_string_list(char *buf,
}
}

+ if (attr == &sysfs_stripe_size) {
+ int v = strtoul_or_return(buf);
+
+ if (v & 0x1FF) {
+ pr_err("stripe_size must be a muliple of 512-byte sectors");
+ return -EINVAL;
+ }
+
+ v >>= 9;
+
+ if (v != dc->sb.stripe_size) {
+ dc->sb.stripe_size = v;
+ pr_info("stripe_size=%d, re-register to take effect.",
+ v<<9);
+ bch_write_bdev_super(dc, NULL);
+ } else
+ pr_info("stripe_size is already set to %d.", v<<9);
+ }
+
+ if (attr == &sysfs_partial_stripes_expensive) {
+ int v = strtoul_or_return(buf);
+
+ if (v != BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb)) {
+ SET_BDEV_PARTIAL_STRIPES_EXPENSIVE(&dc->sb, v);
+ dc->partial_stripes_expensive = v;
+ bch_write_bdev_super(dc, NULL);
+ }
+ }
+
if (attr == &sysfs_stop_when_cache_set_failed) {
v = __sysfs_match_string(bch_stop_on_failure_modes, -1, buf);
if (v < 0)
diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index 5d4f58e..ee60914 100644
--- a/include/uapi/linux/bcache.h
+++ b/include/uapi/linux/bcache.h
@@ -172,7 +172,9 @@ struct cache_sb {

__u64 flags;
__u64 seq;
- __u64 pad[8];
+ __u32 stripe_size;
+ __u32 pad_u32;
+ __u64 pad_u64[7];

union {
struct {
@@ -230,7 +232,7 @@ static inline _Bool SB_IS_BDEV(const struct cache_sb *sb)
#define BDEV_STATE_CLEAN 1U
#define BDEV_STATE_DIRTY 2U
#define BDEV_STATE_STALE 3U
-
+BITMASK(BDEV_PARTIAL_STRIPES_EXPENSIVE, struct cache_sb, flags, 60, 1);
/*
* Magic numbers
*
--
1.8.3.1
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