17726: More detail about singularity/docker differences.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>

diff --git a/doc/install/singularity.html.textile.liquid b/doc/install/singularity.html.textile.liquid
index b3cbe593b60987b32118f4d5612854d65faf7776..121dd86c657b700337917cf67f515889b31d50c3 100644 (file)
--- a/doc/install/singularity.html.textile.liquid
+++ b/doc/install/singularity.html.textile.liquid
@@ -9,13 +9,15 @@ Copyright (C) The Arvados Authors. All rights reserved.
 SPDX-License-Identifier: CC-BY-SA-3.0
 {% endcomment %}
 
-Arvados can be configured to use Singularity instead of Docker to execute containers on cloud nodes or a SLURM/LSF cluster.
+Arvados can be configured to use "Singularity":https://sylabs.io/singularity/ instead of Docker to execute containers on cloud nodes or a SLURM/LSF cluster.
 
 Please note:
 * *Singularity support is currently considered experimental.*
 * Even when using the singularity runtime, users' container images are expected to be saved in Docker format using @arv keep docker@. Arvados converts the Docker image to Singularity format (@.sif@) at runtime as needed. Specifying a @.sif@ file as an image when submitting a container request is not yet supported.
-* Singularity does not limit the amount of memory available in a container. Each container will have access to all memory on the host where it runs.
-* Programs running in containers may behave differently due to differences between Singularity and Docker, _e.g._, the root (image) filesystem is read-only in a Singularity container.
+* Singularity does not limit the amount of memory available in a container. Each container will have access to all memory on the host where it runs, unless memory use is restricted by SLURM/LSF.
+* Programs running in containers may behave differently due to differences between Singularity and Docker.
+** The root (image) filesystem is read-only in a Singularity container. Programs that attempt to write outside a designated output or temporary directory are likely to fail.
+** The Docker ENTRYPOINT instruction is ignored.
 * Arvados is currently tested with Singularity version 3.5.2.
 
 To use singularity, first make sure "Singularity is installed":https://sylabs.io/guides/3.5/user-guide/quick_start.html on your cloud worker image or SLURM/LSF compute nodes as applicable. Note @squashfs-tools@ is required.