diff --git a/.travis.yml b/.travis.yml
index 5901e0b10..ea05101aa 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -30,7 +30,16 @@ notifications:
   slack:
     secure: 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
 
-script:
-- go test -v ./...
-# Another round of tests after turning off mmap
-- go test -v -vlog_mmap=false github.com/dgraph-io/badger
+script: >-
+    if [ $TRAVIS_OS_NAME = "linux" ] && [ $go_32 ]; then
+      uname -a
+      GOOS=linux GOARCH=arm go test -v ./...
+      # Another round of tests after turning off mmap.
+      GOOS=linux GOARCH=arm go test -v -vlog_mmap=false github.com/dgraph-io/badger
+    else
+      go test -v ./...
+      # Another round of tests after turning off mmap.
+      go test -v -vlog_mmap=false github.com/dgraph-io/badger
+      # Cross-compile for Plan 9
+      GOOS=plan9 go build ./...
+    fi
diff --git a/dir_plan9.go b/dir_plan9.go
new file mode 100644
index 000000000..ad323d70a
--- /dev/null
+++ b/dir_plan9.go
@@ -0,0 +1,150 @@
+/*
+ * Copyright 2020 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package badger
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// directoryLockGuard holds a lock on a directory and a pid file inside.  The pid file isn't part
+// of the locking mechanism, it's just advisory.
+type directoryLockGuard struct {
+	// File handle on the directory, which we've locked.
+	f *os.File
+	// The absolute path to our pid file.
+	path string
+}
+
+// acquireDirectoryLock gets a lock on the directory.
+// It will also write our pid to dirPath/pidFileName for convenience.
+// readOnly is not supported on Plan 9.
+func acquireDirectoryLock(dirPath string, pidFileName string, readOnly bool) (
+	*directoryLockGuard, error) {
+	if readOnly {
+		return nil, ErrPlan9NotSupported
+	}
+
+	// Convert to absolute path so that Release still works even if we do an unbalanced
+	// chdir in the meantime.
+	absPidFilePath, err := filepath.Abs(filepath.Join(dirPath, pidFileName))
+	if err != nil {
+		return nil, errors.Wrap(err, "cannot get absolute path for pid lock file")
+	}
+
+	// If the file was unpacked or created by some other program, it might not
+	// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
+	// can be confident that a successful OpenFile implies exclusive use.
+	//
+	// OpenFile fails if the file ModeExclusive bit set *and* the file is already open.
+	// So, if the file is closed when the DB crashed, we're fine. When the process
+	// that was managing the DB crashes, the OS will close the file for us.
+	//
+	// This bit of code is copied from Go's lockedfile internal package:
+	// https://github.com/golang/go/blob/go1.15rc1/src/cmd/go/internal/lockedfile/lockedfile_plan9.go#L58
+	if fi, err := os.Stat(absPidFilePath); err == nil {
+		if fi.Mode()&os.ModeExclusive == 0 {
+			if err := os.Chmod(absPidFilePath, fi.Mode()|os.ModeExclusive); err != nil {
+				return nil, errors.Wrapf(err, "could not set exclusive mode bit")
+			}
+		}
+	} else if !os.IsNotExist(err) {
+		return nil, err
+	}
+	f, err := os.OpenFile(absPidFilePath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0666|os.ModeExclusive)
+	if err != nil {
+		if isLocked(err) {
+			return nil, errors.Wrapf(err,
+				"Cannot open pid lock file %q.  Another process is using this Badger database",
+				absPidFilePath)
+		}
+		return nil, errors.Wrapf(err, "Cannot open pid lock file %q", absPidFilePath)
+	}
+
+	if _, err = fmt.Fprintf(f, "%d\n", os.Getpid()); err != nil {
+		f.Close()
+		return nil, errors.Wrapf(err, "could not write pid")
+	}
+	return &directoryLockGuard{f, absPidFilePath}, nil
+}
+
+// Release deletes the pid file and releases our lock on the directory.
+func (guard *directoryLockGuard) release() error {
+	// It's important that we remove the pid file first.
+	err := os.Remove(guard.path)
+
+	if closeErr := guard.f.Close(); err == nil {
+		err = closeErr
+	}
+	guard.path = ""
+	guard.f = nil
+
+	return err
+}
+
+// openDir opens a directory for syncing.
+func openDir(path string) (*os.File, error) { return os.Open(path) }
+
+// When you create or delete a file, you have to ensure the directory entry for the file is synced
+// in order to guarantee the file is visible (if the system crashes). (See the man page for fsync,
+// or see https://github.com/coreos/etcd/issues/6368 for an example.)
+func syncDir(dir string) error {
+	f, err := openDir(dir)
+	if err != nil {
+		return errors.Wrapf(err, "While opening directory: %s.", dir)
+	}
+
+	err = f.Sync()
+	closeErr := f.Close()
+	if err != nil {
+		return errors.Wrapf(err, "While syncing directory: %s.", dir)
+	}
+	return errors.Wrapf(closeErr, "While closing directory: %s.", dir)
+}
+
+// Opening an exclusive-use file returns an error.
+// The expected error strings are:
+//
+//  - "open/create -- file is locked" (cwfs, kfs)
+//  - "exclusive lock" (fossil)
+//  - "exclusive use file already open" (ramfs)
+//
+// See https://github.com/golang/go/blob/go1.15rc1/src/cmd/go/internal/lockedfile/lockedfile_plan9.go#L16
+var lockedErrStrings = [...]string{
+	"file is locked",
+	"exclusive lock",
+	"exclusive use file already open",
+}
+
+// Even though plan9 doesn't support the Lock/RLock/Unlock functions to
+// manipulate already-open files, IsLocked is still meaningful: os.OpenFile
+// itself may return errors that indicate that a file with the ModeExclusive bit
+// set is already open.
+func isLocked(err error) bool {
+	s := err.Error()
+
+	for _, frag := range lockedErrStrings {
+		if strings.Contains(s, frag) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/dir_unix.go b/dir_unix.go
index d56e6e821..a3fef6697 100644
--- a/dir_unix.go
+++ b/dir_unix.go
@@ -1,4 +1,4 @@
-// +build !windows
+// +build !windows,!plan9
 
 /*
  * Copyright 2017 Dgraph Labs, Inc. and Contributors
diff --git a/errors.go b/errors.go
index 8d2df6833..933cd1307 100644
--- a/errors.go
+++ b/errors.go
@@ -103,6 +103,9 @@ var (
 	// ErrWindowsNotSupported is returned when opt.ReadOnly is used on Windows
 	ErrWindowsNotSupported = errors.New("Read-only mode is not supported on Windows")
 
+	// ErrPlan9NotSupported is returned when opt.ReadOnly is used on Plan 9
+	ErrPlan9NotSupported = errors.New("Read-only mode is not supported on Plan 9")
+
 	// ErrTruncateNeeded is returned when the value log gets corrupt, and requires truncation of
 	// corrupt data to allow Badger to run properly.
 	ErrTruncateNeeded = errors.New(
diff --git a/y/file_dsync.go b/y/file_dsync.go
index 3f3445e2e..ea4d9ab26 100644
--- a/y/file_dsync.go
+++ b/y/file_dsync.go
@@ -1,4 +1,4 @@
-// +build !dragonfly,!freebsd,!windows
+// +build !dragonfly,!freebsd,!windows,!plan9
 
 /*
  * Copyright 2017 Dgraph Labs, Inc. and Contributors
diff --git a/y/file_nodsync.go b/y/file_nodsync.go
index b68be7ab9..54a2184e1 100644
--- a/y/file_nodsync.go
+++ b/y/file_nodsync.go
@@ -1,4 +1,4 @@
-// +build dragonfly freebsd windows
+// +build dragonfly freebsd windows plan9
 
 /*
  * Copyright 2017 Dgraph Labs, Inc. and Contributors
diff --git a/y/mmap_plan9.go b/y/mmap_plan9.go
new file mode 100644
index 000000000..21db76bf0
--- /dev/null
+++ b/y/mmap_plan9.go
@@ -0,0 +1,40 @@
+/*
+ * Copyright 2020 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package y
+
+import (
+	"os"
+	"syscall"
+)
+
+// Mmap uses the mmap system call to memory-map a file. If writable is true,
+// memory protection of the pages is set so that they may be written to as well.
+func mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
+	return nil, syscall.EPLAN9
+}
+
+// Munmap unmaps a previously mapped slice.
+func munmap(b []byte) error {
+	return syscall.EPLAN9
+}
+
+// Madvise uses the madvise system call to give advise about the use of memory
+// when using a slice that is memory-mapped to a file. Set the readahead flag to
+// false if page references are expected in random order.
+func madvise(b []byte, readahead bool) error {
+	return syscall.EPLAN9
+}
diff --git a/y/mmap_unix.go b/y/mmap_unix.go
index 8d2a18ddd..003f59723 100644
--- a/y/mmap_unix.go
+++ b/y/mmap_unix.go
@@ -1,4 +1,4 @@
-// +build !windows,!darwin
+// +build !windows,!darwin,!plan9
 
 /*
  * Copyright 2019 Dgraph Labs, Inc. and Contributors