[PATCH v10 2/3] btrfs: create read policy framework

Anand Jain
2020-10-28T22:41:10Z

As of now, we use the %pid method to read striped mirrored data, which means
process id determines the stripe id to read. This type of routing
typically helps in a system with many small independent processes tying
to read random data. On the other hand, the %pid based read IO policy is
inefficient because if there is a single process trying to read a large
file, the overall disk bandwidth remains under-utilized.

So this patch introduces a read policy framework so that we could add more
read policies, such as IO routing based on the device's wait-queue or manual
when we have a read-preferred device or a policy based on the target
storage caching.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
v10: -
v9: -
v8: use fallthrough;
v7: Fix missing /* fall through */ in the switch
Removed Reviewed-by: Josef Bacik <josef@toxicpanda.com>
v6:-
v5: Title renamed from:- btrfs: add read_policy framework
Change log updated.
Unnecessary comment dropped, added more where necessary.
Optimize code in the switch remove duplicate code.
Define BTRFS_READ_POLICY_DEFAULT dropped.
Rename enum btrfs_read_policy_type to enum btrfs_read_policy.
Rename BTRFS_READ_BY_PID to BTRFS_READ_POLICY_PID.
(As its mainly renames. Reviewed-by retained).
v4: -
v3: Declare fs_devices::readmirror as enum btrfs_readmirror_policy_type
v2: Declare fs_devices::readmirror as u8 instead of atomic_t
A small change in comment and change log wordings.

fs/btrfs/volumes.c | 15 ++++++++++++++-
fs/btrfs/volumes.h | 14 ++++++++++++++
2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index b18e026424a6..6bf487626f23 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1223,6 +1223,7 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
fs_devices->latest_bdev = latest_dev->bdev;
fs_devices->total_rw_bytes = 0;
fs_devices->chunk_alloc_policy = BTRFS_CHUNK_ALLOC_REGULAR;
+ fs_devices->read_policy = BTRFS_READ_POLICY_PID;

return 0;
}
@@ -5485,7 +5486,19 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
else
num_stripes = map->num_stripes;

- preferred_mirror = first + current->pid % num_stripes;
+ switch (fs_info->fs_devices->read_policy) {
+ default:
+ /*
+ * Shouldn't happen, just warn and use pid instead of failing.
+ */
+ btrfs_warn_rl(fs_info,
+ "unknown read_policy type %u, fallback to pid",
+ fs_info->fs_devices->read_policy);
+ fallthrough;
+ case BTRFS_READ_POLICY_PID:
+ preferred_mirror = first + current->pid % num_stripes;
+ break;
+ }

if (dev_replace_is_ongoing &&
fs_info->dev_replace.cont_reading_from_srcdev_mode ==
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 232f02bd214f..97f075516696 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -211,6 +211,15 @@ enum btrfs_chunk_allocation_policy {
BTRFS_CHUNK_ALLOC_REGULAR,
};

+/*
+ * Read policies for the mirrored block groups, read picks the stripe based
+ * on these policies.
+ */
+enum btrfs_read_policy {
+ BTRFS_READ_POLICY_PID,
+ BTRFS_NR_READ_POLICY,
+};
+
struct btrfs_fs_devices {
u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
u8 metadata_uuid[BTRFS_FSID_SIZE];
@@ -264,6 +273,11 @@ struct btrfs_fs_devices {
struct completion kobj_unregister;

enum btrfs_chunk_allocation_policy chunk_alloc_policy;
+
+ /*
+ * policy used to read the mirrored stripes
+ */
+ enum btrfs_read_policy read_policy;
};

#define BTRFS_BIO_INLINE_CSUM_SIZE 64
--
2.25.1