@@ -5665,6 +5665,12 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
fs_info->fs_devices->read_policy = BTRFS_READ_POLICY_PID;
fallthrough;
case BTRFS_READ_POLICY_PID:
+ /*
+ * Just to factor in the cost of calculating the avg wait using
+ * iostat, call btrfs_find_best_stripe() here for the PID policy
+ * and drop its results on the floor.
+ */
+ btrfs_find_best_stripe(fs_info, map, first, num_stripes, log,
+ logsz);
preferred_mirror = first + current->pid % num_stripes;
scnprintf(log, logsz,
"first %d num_stripe %d %s (%d) preferred %d",
v3: The block layer commit 0d02129e76ed (block: merge struct block_device and struct hd_struct) has changed the first argument in the function part_stat_read_all() in 5.11-rc1. So trickle down its changes in the patch 1/4. v2: Fixes as per review comments, as in the individual patches. v1: Drop tracing patch Drop factoring inflight command Here below is the performance differences, when inflight is used, it pushed few commands to the other device, so losing the potential merges. Few C styles fixes. ----- This patchset adds read policy types latency, device, and round-robin, for the mirrored raid profiles such as raid1, raid1c3, raid1c4, and raid10. The default read policy remains as PID, as of now. Read policy types: Latency: Latency policy routes the read IO based on the historical average wait time experienced by the read IOs on the individual device. Device: With the device policy along with the read_preferred flag, you can set the device for reading manually. Useful to test mirrors in a deterministic way and helps advance system administrations. Round-robin (RFC patch): Alternates striped device in a round-robin loop for reading. To achieve this first we put the stripes in an array, sort it by devid and pick the next device. Test scripts: ============= I have included a few scripts which were useful for testing. -------------------8<-------------------------------- Set latency policy on the btrfs mounted at /mnt Usage example: $ readpolicyset /mnt latency $ cat readpolicyset #!/bin/bash : ${1?"arg1 <mnt> missing"} : ${2?"arg2 <pid|latency|device|roundrobin> missing"} mnt=$1 policy=$2 [ $policy == "device" ] && { : ${3?"arg3 <devid> missing"}; } devid=$3 uuid=$(btrfs fi show -m /btrfs | grep uuid | awk '{print $4}') p=/sys/fs/btrfs/$uuid/read_policy q=/sys/fs/btrfs/$uuid/devinfo [ $policy == "device" ] && { echo 1 > ${q}/$devid/read_preferred || exit $?; } echo $policy > $p exit $? -------------------8<-------------------------------- Read policy type from the btrfs mounted at /mnt Usage example: $ readpolicy /mnt $ cat readpolicy #!/bin/bash : ${1?"arg1 <mnt> missing"} mnt=$1 uuid=$(btrfs fi show -m /btrfs | grep uuid | awk '{print $4}') p=/sys/fs/btrfs/$uuid/read_policy q=/sys/fs/btrfs/$uuid/devinfo policy=$(cat $p) echo -n "$policy ( " for i in $(find $q -type f -name read_preferred | xargs cat) do echo -n "$i" done echo ")" -------------------8<-------------------------------- Show the number of read IO per devices for the give command. Usage example: $ readstat /mnt fioread $ cat readstat #!/bin/bash : ${1?"arg1 <mnt> is missing"} : ${2?"arg2 <cmd-to-run> is missing"} mnt=$1; shift mountpoint -q $mnt || { echo "ERROR: $mnt is not mounted"; exit 1; } declare -A devread for dev in $(btrfs filesystem show -m $mnt | grep devid |awk '{print $8}') do prefix=$(echo $dev | rev | cut -d"/" -f1 | rev) sysfs_path=$(find /sys | grep $prefix/stat$) devread[$sysfs_path]=$(cat $sysfs_path | awk '{print $1}') done "$@" | grep "READ: bw" echo echo for sysfs_path in ${!devread[@]} do dev=$(echo $sysfs_path | rev | cut -d"/" -f2 | rev) new=$(cat $sysfs_path | awk '{print $1}') old=${devread[$sysfs_path]} echo "$dev $((new - old))" done -------------------8<-------------------------------- Run fio read command Usage example: $ touch /mnt/largefile $ fioread /mnt/largefile 500m $ cat fioread #!/bin/bash : ${1?"arg1 </mnt/file> is missing"} : ${2?"arg2 <1Gi|50Gi> is missing"} tf=$1 sz=$2 mnt=$(stat -c '%m' $tf) fio \ --filename=$tf \ --directory=$mnt \ --filesize=$sz \ --size=$sz \ --rw=randread \ --bs=64k \ --ioengine=libaio \ --direct=1 \ --numjobs=32 \ --group_reporting \ --thread \ --name iops-test-job -------------------8<-------------------------------- Testing on guest VM ~~~~~~~~~~~~~~~~~~~ The test results from my VM with 2 devices of type sata and 2 devices of type virtio, are below. Performance results are for raid1c4, raid10, and raid1 are as below. The workload is fio read 32 threads, 500m random reads. Fio is passed to the script called readstat, which returns the number of read IOs per device sent during the fio. Supporting fio logs are below. And readstat shows the number of read IOs to the devices (excluding the merges). raid1c4 ======= pid ---- $ readpolicyset /btrfs pid && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m [pid] latency device roundrobin ( 0000) READ: bw=87.0MiB/s (91.2MB/s), 87.0MiB/s-87.0MiB/s (91.2MB/s-91.2MB/s), io=15.6GiB (16.8GB), run=183884-183884msec vdb 64060 vdc 64053 sdb 64072 sda 64054 latency ------- (All devices are non-rotational, but sda and sdb are of type sata and vdb and vdc are of type virtio). $ readpolicyset /btrfs latency && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid [latency] device roundrobin ( 0000) READ: bw=87.1MiB/s (91.3MB/s), 87.1MiB/s-87.1MiB/s (91.3MB/s-91.3MB/s), io=15.6GiB (16.8GB), run=183774-183774msec vdb 255844 vdc 559 sdb 0 sda 93 roundrobin ---------- $ readpolicyset /btrfs roundrobin && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid latency device [roundrobin] ( 0000) READ: bw=51.0MiB/s (54.5MB/s), 51.0MiB/s-51.0MiB/s (54.5MB/s-54.5MB/s), io=15.6GiB (16.8GB), run=307755-307755msec vdb 866859 vdc 866651 sdb 864139 sda 865533 raid10 ====== pid --- $ readpolicyset /btrfs pid && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m [pid] latency device roundrobin ( 0000) READ: bw=85.2MiB/s (89.3MB/s), 85.2MiB/s-85.2MiB/s (89.3MB/s-89.3MB/s), io=15.6GiB (16.8GB), run=187864-187864msec sdf 64053 sde 64036 sdd 64043 sdc 64038 latency ------- $ readpolicyset /btrfs latency && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid [latency] device roundrobin ( 0000) READ: bw=85.4MiB/s (89.5MB/s), 85.4MiB/s-85.4MiB/s (89.5MB/s-89.5MB/s), io=15.6GiB (16.8GB), run=187370-187370msec sdf 117494 sde 10748 sdd 125247 sdc 2921 roundrobin ---------- $ readpolicyset /btrfs roundrobin && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid latency device [roundrobin] ( 0000) READ: bw=55.4MiB/s (58.1MB/s), 55.4MiB/s-55.4MiB/s (58.1MB/s-58.1MB/s), io=15.6GiB (16.8GB), run=288701-288701msec sdf 617593 sde 617381 sdd 618486 sdc 618633 raid1 ===== pid ---- $ readpolicyset /btrfs pid && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m [pid] latency device roundrobin ( 00) READ: bw=78.8MiB/s (82.6MB/s), 78.8MiB/s-78.8MiB/s (82.6MB/s-82.6MB/s), io=15.6GiB (16.8GB), run=203158-203158msec sdb 128087 sda 128090 latency -------- $ readpolicyset /btrfs latency && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid [latency] device roundrobin ( 00) READ: bw=86.5MiB/s (90.7MB/s), 86.5MiB/s-86.5MiB/s (90.7MB/s-90.7MB/s), io=15.6GiB (16.8GB), run=185023-185023msec sdb 567 sda 255942 device ------- (From the latency test results (above) we know sda is providing low latency read IO. So set sda as read preferred device.) $ readpolicyset /btrfs device 1 && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid latency [device] roundrobin ( 10) READ: bw=88.2MiB/s (92.5MB/s), 88.2MiB/s-88.2MiB/s (92.5MB/s-92.5MB/s), io=15.6GiB (16.8GB), run=181374-181374msec sdb 0 sda 256191 roundrobin ----------- $ readpolicyset /btrfs roundrobin && readpolicy /btrfs && dropcache && readstat /btrfs fioread /btrfs/largefile 500m pid latency device [roundrobin] ( 00) READ: bw=54.1MiB/s (56.7MB/s), 54.1MiB/s-54.1MiB/s (56.7MB/s-56.7MB/s), io=15.6GiB (16.8GB), run=295693-295693msec sdb 1252584 sda 1254258 Testing on real hardware: ~~~~~~~~~~~~~~~~~~~~~~~~ raid1 Read 500m ----------------------------------------------------- |nvme+ssd nvme+ssd all-nvme all-nvme |random sequential random sequential ------------+------------------------------------------ pid | 744MiB/s 809MiB/s 2225MiB/s 2155MiB/s latency |2072MiB/s 2008MiB/s 1999MiB/s 1961MiB/s device(nvme)|2187MiB/s 2063MiB/s 2125MiB/s 2080MiB/s roundrobin | 527MiB/s 519MiB/s 2137MiB/s 1876MiB/s raid10 Read 500m ----------------------------------------------------- | nvme+ssd nvme+ssd all-nvme all-nvme | random seq random seq ------------+----------------------------------------- pid | 1282MiB/s 1427MiB/s 2152MiB/s 1969MiB/s latency | 2073MiB/s 1871MiB/s 1975MiB/s 1984MiB/s device(nvme)| 2447MiB/s 1873MiB/s 2184MiB/s 2015MiB/s roundrobin | 1117MiB/s 1076MiB/s 2020MiB/s 2030MiB/s raid1c3 Read 500m ----------------------------------------------------- | nvme+ssd nvme+ssd all-nvme all-nvme | random seq random seq ------------+----------------------------------------- pid | 973MiB/s 955MiB/s 2144MiB/s 1962MiB/s latency | 2005MiB/s 1924MiB/s 2083MiB/s 1980MiB/s device(nvme)| 2021MiB/s 2034MiB/s 1920MiB/s 2132MiB/s roundrobin | 707MiB/s 701MiB/s 1760MiB/s 1990MiB/s raid1c4 Read 500m ----------------------------------------------------- | nvme+ssd nvme+ssd all-nvme all-nvme | random seq random seq ------------+---------------------------------------- pid | 1204MiB/s 1221MiB/s 2065MiB/s 1878MiB/s latency | 1990MiB/s 1920MiB/s 1945MiB/s 1865MiB/s device(nvme)| 2109MiB/s 1935MiB/s 2153MiB/s 1991MiB/s roundrobin | 887MiB/s 865MiB/s 1948MiB/s 1796MiB/s Observations: ============= 1. As our chunk allocation is based on the device's available size at that time. So stripe 0 may be circulating among the devices. So a single-threaded process running with a constant PID, may balance the read IO among devices. But it is not guaranteed to work in all the cases, and it might not work very well in the case of raid1c3/4. Further, PID provides terrible performance if the devices are heterogeneous in terms of either type, speed, or size. 2. Latency provides performance equal to PID if all devices are of same type. Latency needs iostat be enabled and includes cost of calculating the avg. wait time. So if you factor in a similar cost of calculating the avg. wait time in case of PID policy (using the debug code [2]) then the Latency performance is better than PID. This proves that read IO distribution as per latency is working, but there is a cost to it. And moreover, latency works for any type of devices. 3. Round robin is worst (unless there is a bug in my patch). The total number of new IOs issued is almost double when compared with the PID and Latency read_policy, that's because there were fewer number of IO merges in the block layer due to constant switching of devices in the btrfs. 4. 4. Device read_policy is useful in testing and provides advanced sysadmin capabilities. When known how to use, the policy could help avert performance degradation due to csum/IO errors at production. Thanks, Anand ------------------ [2] Debug patch to factor the cost of calculating the latency per IO. Anand Jain (4): btrfs: add read_policy latency btrfs: introduce new device-state read_preferred btrfs: introduce new read_policy device btrfs: introduce new read_policy round-robin fs/btrfs/sysfs.c | 57 ++++++++++++++++++++++- fs/btrfs/volumes.c | 110 +++++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/volumes.h | 8 ++++ 3 files changed, 174 insertions(+), 1 deletion(-)