One of the more controversial topics right now in the Docker community is the issue surrounding file system performance in the Docker for Mac application.
For a very long time users have been forced to use workarounds to speed up performance when dealing with slow read and write times. For example, this thread has been open on the Docker forums for over a year now, describing the problem and various workarounds users have found during that time. There have been blog posts describing various optimizations, as well as scripts and tools to alleviate some of the frustration around slow file system performance on Docker for Mac.
There is a great explanation from the Docker team that lays out the details of the file system performance issues and what the crux of the problem is right now.
At the highest level, there are two dimensions to file system performance: throughput (read/write IO) and latency (roundtrip time). In a traditional file system on a modern SSD, applications can generally expect throughput of a few GB/s. With large sequential IO operations,
osxfscan achieve throughput of around 250 MB/s which, while not native speed, will not be the bottleneck for most applications which perform acceptably on HDDs.
The article later goes on to highlight the plan to improve performance along with a number of specific items for accomplishing this.
Under development, we have:
- A Linux kernel patch to reduce data path latency by 2/7 copies and 2/5 context switches
- Increased OS X integration to reduce the latency between the hypervisor and the file system server
- A server-side directory read cache to speed up traversal of large directories
- User-facing file system tracing capabilities so that you can send us recordings of slow workloads for analysis
- A growing performance test suite of real world use cases (more on this below in What you can do)
- Experimental support for using Linux’s inode, writeback, and page caches
- End-user controls to configure the coherence of subsets of cross-OS bind mounts without exposing all of the underlying complexity
Additionally, with the latest release of the Docker for Mac 17.04-ce-mac7 (April 6 2017) client, a new :cached flag has been introduced for volume mounts to help with read times for lots of files. There is also work going on to introduce another :delegated flag to help speed up write times.
Initial user testing of the :cached flag has been good, and shown up to a 4x improvement in some cases. You can follow this issue on Github to get the most up to date information. There is some really good detail and discussion going on over there (towards the bottom of the issue is where the new flags are discussed).
Overall I think Docker has done a great job of keeping users informed and updated on the various aspects of the problem and has been steadily making progress in addressing the situation. The container ecosystem is still very young so there will be growing pains along the way and I think the way that Docker has been handling things has been more than reasonable as they have consistently been making progress on addressing the issue and have been transparent in recent months about what’s going on and how they’re working on the problem.
