sbt plugin to speed up development by leveraging tmpfs.
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fileSyncTest Update May 26, 2017

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sbt plugin that automatically "tmpfsifies" directories to speed up development.

sbt 1.0.0+. For those of you who have trouble downloading doc and src through IDEA, use reload plugins then updateClassifiers in console.

Why tmpfs?

Comparison with common specs:

Tpe Read speed Response time Power usage
ram 30GB/s 100 ns 2 - 3 w
ssd 500MB/s 0.1 ms 1 - 3 w
sas-hdd fast in raid 20+ ms 15 w

RAM is thousands of times faster than SSD. Tmpfs provides an easy way to leverage RAM. More, RAM is immune to the exertion brought by infinite compilation/clean cycles. While SSD is tolerant, developers are not. Latency Numbers Every Programmer Should Know

Why sbt-tmpfs?

sbt-tmpfs brings automation to leverage tmpfs to speedup your development.

How to use:

Note: due to #1444, make sure sbt >= 0.13.14

Mount your /tmp with tmpfs, by adding this line to your /etc/fstab if you haven't yet:

tmpfs /tmp tmpfs rw,nosuid,nodev,noatime

Reboot you pc.

Add below to project/plugins.sbt:

addSbtPlugin("com.github.cuzfrog" % "sbt-tmpfs" % "0.3.4") //for sbt 1.1 and sbt 0.13

Now, enjoy RAM speed!

(If you use sbt-revolver reStart, run compile to trigger sbt-tmpfs first.)

Detail and Configuration:

Directory mode:

There are 2 strategies to use tmpfs:

  • Symlink into existing tmpfs dir. (Harder management.)
  • Directly mount the point with tmpfs. (Requires super privilege)

Symlink is the default one, since it does not require super. Default linking dirs include crossTarget and target/resolution-cache. Add more dirs to be linked:

tmpfsLinkDirectories ++= Seq(//your dirs here.)

Broken symlink will be overwrite by sbt-tmpfs. (Symlink of some dir like streams may lead to sbt error when symlink is broken after a reboot.) The base tmpfs dir where symlinks point to, by default, is /tmp, which is controlled by tmpfsLinkBaseDirectory.

When Mount mode is in use, sbt command line may require super password to execute shell command. Mount size limit key:tmpfsMountSizeLimit , shell command can also be changed by tmpfsMountCommand. Default mount point is target. Add more dirs to be mounted:

tmpfsMountDirectories ++= Seq(//your dirs here.)

In fact, Mount mode is recommended. It's easier to handle in most cases, and not likely to cause some unexpectation.

You can set below in your build.sbt.

tmpfsDirectoryMode := TmpfsDirectoryMode.Mount
onLoad in Global := {
  val insertCommand: State => State =
    (state: State) =>
      state.copy(remainingCommands = Exec(";project1/tmpfsOn;project2/tmpfsOn", None) +: state.remainingCommands)
  (onLoad in Global).value andThen insertCommand

Changing mode after the other has been done, will cause some minor inconsistency. For example: if target has been mounted first, tmpfsLink task may have no effect. It will realize that dirs inside target are all of tmpfs now, so it aborts linking. Fortunately, most of the inconsistency will be repaired after a reboot or clean.

Work flow:

sbt-tmpfs checks target dirs defined in key tmpfsLinkDirectories or tmpfsMountDirectories and mounts/links tmpfs when necessary.

Under Symlink mode, when user does a clean, symlinks themselves will be purged. When new symlinks are created, sbt-tmpfs deletes old dirs in tmpfs that old symlinks referenced.

Task tmpfsOn: check and link/mount when needed. Dyn-defined by mode as tmpfsLink or tmpfsMount.

Task tmpfsLink: check and link when needed. runBefore update, triggeredBy clean.

Task tmpfsMount: check and mount when needed.

Task tmpfsSyncMapping: sync mapped dirs, triggered by above.

On initializing, sbt-tmpfs will try to clean dead(broken) symlinks, possibly created last time.

Map and sync dirs:

Sometimes, we want to speedup some dirs while wanting to preserve them on disk, like node_modules, we can map these dirs.

tmpfsMappingDirectories := Map(
  sourceDir -> Seq(destDir) //sourceDir is somewhere on disk.

sbt-tmpfs will link/mount destDir with tmpfs, if they are not an active symlink or already of tmpfs, and automatically does one-way-synchronization: from source to destination.

There is an Interesting Test: sbt.IO-vs-rsync-vs-cp about choosing which method to do the sync.

destDirs are added to cleanKeepFiles by sbt-tmpfs automatically (but only direct children of target take effect).

Debug info:

sbt-tmpfs has thorough debug log. Set log level to debug in tasks respectively:

logLevel in tmpfsOn := Level.Debug
logLevel in tmpfsLink := Level.Debug
logLevel in tmpfsMount := Level.Debug
logLevel in tmpfsSyncMapping := Level.Debug


Author: Cause Chung (

License: Apache License Version 2.0