This document is for a development version of Ceph.


Crimson is the code name of crimson-osd, which is the next generation ceph-osd. It targets fast networking devices, fast storage devices by leveraging state of the art technologies like DPDK and SPDK, for better performance. And it will keep the support of HDDs and low-end SSDs via BlueStore. Crismon will try to be backward compatible with classic OSD.

Building Crimson

Crismon is not enabled by default. To enable it:

$ WITH_SEASTAR=true ./
$ mkdir build && cd build
$ cmake -DWITH_SEASTAR=ON ..

Please note, ASan is enabled by default if crimson is built from a source cloned using git.

Also, Seastar uses its own lockless allocator which does not play well with the alien threads. So, to use alienstore / bluestore backend, you might want to pass -DSeastar_CXX_FLAGS=-DSEASTAR_DEFAULT_ALLOCATOR to cmake when configuring this project to use the libc allocator, like:


Running Crimson

As you might expect, crimson is not featurewise on par with its predecessor yet.

object store backend

At the moment crimson-osd offers two object store backends:

  • CyanStore: CyanStore is modeled after memstore in classic OSD.

  • AlienStore: AlienStore is short for Alienized BlueStore.

Seastore is still under active development.


Unlike ceph-osd, crimson-osd does daemonize itself even if the daemonize option is enabled. Because, to read this option, crimson-osd needs to ready its config sharded service, but this sharded service lives in the seastar reactor. If we fork a child process and exit the parent after starting the Seastar engine, that will leave us with a single thread which is the replica of the thread calls fork(). This would unnecessarily complicate the code, if we would have tackled this problem in crimson.

Since a lot of GNU/Linux distros are using systemd nowadays, which is able to daemonize the application, there is no need to daemonize by ourselves. For those who are using sysvinit, they can use start-stop-daemon for daemonizing crimson-osd. If this is not acceptable, we can whip up a helper utility to do the trick.


Currently, crimson-osd uses the logging utility offered by Seastar. see src/common/dout.h for the mapping between different logging levels to the severity levels in Seastar. For instance, the messages sent to derr will be printed using logger::error(), and the messages with debug level over 20 will be printed using logger::trace().



< 0




[1, 5)


[5, 20]


> 20


Please note, crimson-osd does not send the logging message to specified log_file. It writes the logging messages to stdout and/or syslog. Again, this behavior can be changed using --log-to-stdout and --log-to-syslog command line options. By default, log-to-stdout is enabled, and the latter disabled.

To facilitate the development of crimson, following options would be handy when using,


start crimson-osd instead of ceph-osd


do not daemonize the service


redirect the stdout and stderr of service to out/$type.$num.stdout.


pass extra command line options to crimson-osd or ceph-osd. It’s quite useful for passing Seastar options to crimson-osd. For instance, you could use --osd-args "--memory 2G" to set the memory to use. Please refer the output of:

crimson-osd --help-seastar

for more Seastar specific command line options.


use the CyanStore as the object store backend.


use the AlienStore as the object store backend. This is the default setting, if not specified otherwise.

So, a typical command to start a single-crimson-node cluster is:

$  MGR=1 MON=1 OSD=1 MDS=0 RGW=0 ../src/ -n -x \
  --without-dashboard --memstore \
  --crimson --nodaemon --redirect-output \
  --osd-args "--memory 4G"

Where we assign 4 GiB memory, a single thread running on core-0 to crimson-osd.

You could stop the vstart cluster using:

$ ../src/ --crimson

CBT Based Testing

We can use cbt for performing perf tests:

$ git checkout master
$ make crimson-osd
$ ../src/script/ --cbt ~/dev/cbt -a /tmp/baseline ../src/test/crimson/cbt/radosbench_4K_read.yaml
$ git checkout yet-another-pr
$ make crimson-osd
$ ../src/script/ --cbt ~/dev/cbt -a /tmp/yap ../src/test/crimson/cbt/radosbench_4K_read.yaml
$ ~/dev/cbt/ -b /tmp/baseline -a /tmp/yap -v
19:48:23 - INFO     - cbt      - prefill/gen8/0: bandwidth: (or (greater) (near 0.05)):: 0.183165/0.186155  => accepted
19:48:23 - INFO     - cbt      - prefill/gen8/0: iops_avg: (or (greater) (near 0.05)):: 46.0/47.0  => accepted
19:48:23 - WARNING  - cbt      - prefill/gen8/0: iops_stddev: (or (less) (near 0.05)):: 10.4403/6.65833  => rejected
19:48:23 - INFO     - cbt      - prefill/gen8/0: latency_avg: (or (less) (near 0.05)):: 0.340868/0.333712  => accepted
19:48:23 - INFO     - cbt      - prefill/gen8/1: bandwidth: (or (greater) (near 0.05)):: 0.190447/0.177619  => accepted
19:48:23 - INFO     - cbt      - prefill/gen8/1: iops_avg: (or (greater) (near 0.05)):: 48.0/45.0  => accepted
19:48:23 - INFO     - cbt      - prefill/gen8/1: iops_stddev: (or (less) (near 0.05)):: 6.1101/9.81495  => accepted
19:48:23 - INFO     - cbt      - prefill/gen8/1: latency_avg: (or (less) (near 0.05)):: 0.325163/0.350251  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/0: bandwidth: (or (greater) (near 0.05)):: 1.24654/1.22336  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/0: iops_avg: (or (greater) (near 0.05)):: 319.0/313.0  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/0: iops_stddev: (or (less) (near 0.05)):: 0.0/0.0  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/0: latency_avg: (or (less) (near 0.05)):: 0.0497733/0.0509029  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/1: bandwidth: (or (greater) (near 0.05)):: 1.22717/1.11372  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/1: iops_avg: (or (greater) (near 0.05)):: 314.0/285.0  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/1: iops_stddev: (or (less) (near 0.05)):: 0.0/0.0  => accepted
19:48:23 - INFO     - cbt      - seq/gen8/1: latency_avg: (or (less) (near 0.05)):: 0.0508262/0.0557337  => accepted
19:48:23 - WARNING  - cbt      - 1 tests failed out of 16

Where we compile and run the same test against two branches. One is master, another is yet-another-pr branch. And then we compare the test results. Along with every test case, a set of rules is defined to check if we have performance regressions when comparing two set of test results. If a possible regression is found, the rule and corresponding test results are highlighted.

Hacking Crimson

Seastar Documents

See Seastar Tutorial . Or build a browsable version and start an HTTP server:

$ cd seastar
$ ./ --mode debug
$ ninja -C build/debug docs
$ python3 -m http.server -d build/debug/doc/html

You might want to install pandoc and other dependencies beforehand.

Debugging Crimson

Debugging with GDB

The tips for debugging Scylla also apply to Crimson.

Human-readable backtraces with addr2line

When a seastar application crashes, it leaves us with a serial of addresses, like:

Segmentation fault.
Segmentation fault

seastar-addr2line offered by Seastar can be used to decipher these addresses. After running the script, it will be waiting for input from stdin, so we need to copy and paste the above addresses, then send the EOF by inputting control-D in the terminal:

$ ../src/seastar/scripts/seastar-addr2line -e bin/crimson-osd

[Backtrace #0]
seastar::backtrace_buffer::append_backtrace() at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::print_with_backtrace(seastar::backtrace_buffer&) at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::print_with_backtrace(char const*) at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::sigsegv_action() at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::install_oneshot_signal_handler<11, &seastar::sigsegv_action>()::{lambda(int, siginfo_t*, void*)#1}::operator()(int, siginfo_t*, void*) const at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::install_oneshot_signal_handler<11, &seastar::sigsegv_action>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) at /home/kefu/dev/ceph/build/../src/seastar/src/core/
?? ??:0
seastar::smp::configure(boost::program_options::variables_map, seastar::reactor_config) at /home/kefu/dev/ceph/build/../src/seastar/src/core/
seastar::app_template::run_deprecated(int, char**, std::function<void ()>&&) at /home/kefu/dev/ceph/build/../src/seastar/src/core/ (discriminator 5)
main at /home/kefu/dev/ceph/build/../src/crimson/osd/ (discriminator 1)

Please note, seastar-addr2line is able to extract the addresses from the input, so you can also paste the log messages like:

2020-07-22T11:37:04.500  0x0000000000e78dbc
2020-07-22T11:37:04.501  0x0000000000e3e7f0
2020-07-22T11:37:04.501  0x0000000000e3e8b8
2020-07-22T11:37:04.501  0x0000000000e3e985
2020-07-22T11:37:04.501  /lib64/

Unlike classic OSD, crimson does not print a human-readable backtrace when it handles fatal signals like SIGSEGV or SIGABRT. And it is more complicated when it comes to a stripped binary. So before planting a signal handler for those signals in crimson, we could to use script/ to parse the addresses in the backtrace:

# assuming you are under the source tree of ceph
$ ./src/script/  --flavor crimson master:27e237c137c330ebb82627166927b7681b20d0aa centos:8
[root@3deb50a8ad51 ~]# wget -q
[root@3deb50a8ad51 ~]# dnf install -q -y file
[root@3deb50a8ad51 ~]# python3 seastar-addr2line -e /usr/bin/crimson-osd
# paste the backtrace here