"Error: unknown" during boot free disk space
bug
### Describe the bug
RabbitMQ 4.2.5 on Arch Linux is failing to boot, timing out while free disk space checks are failing. The logs suggest it has guessed a total amount of RAM (there is in fact 8GB) but it then appears to error out because of it.
I'm ready to upgrade to 4.3.x, but there are unapplied feature flags and I can't get in to enable them.
### Reproduction steps
Start RabbitMQ using the packaged systemd service: `sudo systemctl start rabbitmq`
### Expected behavior
Successful startup.
### Additional context
<details>
<summary>Logs</summary>
```
2026-05-30 11:56:45.464085+01:00 [notice] <0.211.0> Logging: configured log handlers are now ACTIVE
2026-05-30 11:56:45.468075+01:00 [info] <0.211.0> ra: starting system coordination
2026-05-30 11:56:45.468155+01:00 [info] <0.211.0> starting Ra system: coordination in directory: /var/lib/rabbitmq/mnesia/rabbit@localhost/coordination/rabbit@localhost
2026-05-30 11:56:45.471640+01:00 [info] <0.220.0> ra_coordination_log_ets: in system coordination initialising. Mem table opts: [set,{write_concurrency,auto},public,{compressed,false}]
2026-05-30 11:56:45.499432+01:00 [info] <0.226.0> ra system 'coordination' running pre init for 1 registered servers
2026-05-30 11:56:45.510599+01:00 [info] <0.227.0> ra: meta data store initialised for system coordination. 1 record(s) recovered
2026-05-30 11:56:45.518157+01:00 [notice] <0.232.0> WAL: ra_coordination_log_wal init, mem-tables table name: ra_coordination_log_open_mem_tables
2026-05-30 11:56:45.537628+01:00 [info] <0.211.0> ra: starting system quorum_queues
2026-05-30 11:56:45.537677+01:00 [info] <0.211.0> starting Ra system: quorum_queues in directory: /var/lib/rabbitmq/mnesia/rabbit@localhost/quorum/rabbit@localhost
2026-05-30 11:56:45.537829+01:00 [info] <0.237.0> ra_log_ets: in system quorum_queues initialising. Mem table opts: [set,{write_concurrency,auto},public,{compressed,true}]
2026-05-30 11:56:45.538414+01:00 [info] <0.241.0> ra system 'quorum_queues' running pre init for 0 registered servers
2026-05-30 11:56:45.538842+01:00 [info] <0.242.0> ra: meta data store initialised for system quorum_queues. 0 record(s) recovered
2026-05-30 11:56:45.538984+01:00 [notice] <0.247.0> WAL: ra_log_wal init, mem-tables table name: ra_log_open_mem_tables
2026-05-30 11:56:45.543873+01:00 [info] <0.249.0> ra_system_recover: ra system 'quorum_queues' server recovery strategy rabbit_quorum_queue:system_recover
2026-05-30 11:56:45.543971+01:00 [info] <0.249.0> [rabbit_quorum_queue:system_recover/1] rabbit not booted, skipping queue recovery
2026-05-30 11:56:45.544079+01:00 [info] <0.211.0> ra: starting system coordination
2026-05-30 11:56:45.544108+01:00 [info] <0.211.0> starting Ra system: coordination in directory: /var/lib/rabbitmq/mnesia/rabbit@localhost/coordination/rabbit@localhost
2026-05-30 11:56:46.132763+01:00 [notice] <0.251.0> RabbitMQ metadata store: candidate -> leader in term: 24 machine version: 2, last applied 1079
2026-05-30 11:56:46.134808+01:00 [info] <0.211.0>
2026-05-30 11:56:46.134808+01:00 [info] <0.211.0> Starting RabbitMQ 4.2.5 on Erlang 29.0 [jit]
2026-05-30 11:56:46.134808+01:00 [info] <0.211.0> Copyright (c) 2007-2026 Broadcom Inc and/or its subsidiaries
2026-05-30 11:56:46.134808+01:00 [info] <0.211.0> Licensed under the MPL 2.0. Website: https://rabbitmq.com
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0>
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> node : rabbit@localhost
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> home dir : /var/lib/rabbitmq
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> config file(s) : (none)
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> cookie hash : 54cyGV4/EM5OiWTKrSt6+Q==
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> log(s) : /var/log/rabbitmq/rabbit@localhost.log
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> : <stdout>
2026-05-30 11:56:46.140548+01:00 [info] <0.211.0> data dir : /var/lib/rabbitmq/mnesia/rabbit@localhost
2026-05-30 11:56:46.425591+01:00 [info] <0.211.0> Running boot step pre_boot defined by app rabbit
2026-05-30 11:56:46.425675+01:00 [info] <0.211.0> Running boot step rabbit_global_counters defined by app rabbit
2026-05-30 11:56:46.426069+01:00 [info] <0.211.0> Running boot step rabbit_osiris_metrics defined by app rabbit
2026-05-30 11:56:46.426137+01:00 [info] <0.211.0> Running boot step rabbit_core_metrics defined by app rabbit
2026-05-30 11:56:46.426444+01:00 [info] <0.211.0> Running boot step rabbit_alarm defined by app rabbit
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> Failed to get total system memory:
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {badmatch,{error,enoent}}
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> [{vm_memory_monitor,read_proc_file,1,
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> [{file,"vm_memory_monitor.erl"},{line,596}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {vm_memory_monitor,get_total_memory,1,
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> [{file,"vm_memory_monitor.erl"},{line,510}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {vm_memory_monitor,get_total_memory_from_os,0,
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> [{file,"vm_memory_monitor.erl"},{line,308}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {vm_memory_monitor,set_mem_limits,2,
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> [{file,"vm_memory_monitor.erl"},{line,329}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {vm_memory_monitor,init,1,[{file,"vm_memory_monitor.erl"},{line,195}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {gen_server,init_it,2,[{file,"gen_server.erl"},{line,2276}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {gen_server,init_it,6,[{file,"gen_server.erl"},{line,2236}]},
2026-05-30 11:56:46.434803+01:00 [warning] <0.265.0> {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,333}]}]
2026-05-30 11:56:46.435002+01:00 [warning] <0.265.0> Unknown total memory size for your OS {unix,linux}. Assuming memory size is 1024 MiB (1073741824 bytes).
2026-05-30 11:56:46.435078+01:00 [info] <0.265.0> Memory high watermark set to 614 MiB (644245094 bytes) of 1024 MiB (1073741824 bytes) total
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> Failed to get total system memory:
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {badmatch,{error,enoent}}
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> [{vm_memory_monitor,read_proc_file,1,
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,596}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory,1,
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,510}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory_from_os,0,
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,308}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {rabbit_disk_monitor,enable_handle_disk_free,2,
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> [{file,"rabbit_disk_monitor.erl"},{line,444}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {rabbit_disk_monitor,init,1,[{file,"rabbit_disk_monitor.erl"},{line,146}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {gen_server,init_it,2,[{file,"gen_server.erl"},{line,2276}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {gen_server,init_it,6,[{file,"gen_server.erl"},{line,2236}]},
2026-05-30 11:56:46.438683+01:00 [warning] <0.267.0> {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,333}]}]
2026-05-30 11:56:46.438870+01:00 [warning] <0.267.0> Free disk space monitor encountered an error retrieving total memory. Retries left: 10 Error:
2026-05-30 11:56:46.438870+01:00 [warning] <0.267.0> unknown
2026-05-30 11:56:46.438963+01:00 [info] <0.211.0> Running boot step code_server_cache defined by app rabbit
2026-05-30 11:56:46.439031+01:00 [info] <0.211.0> Running boot step file_handle_cache defined by app rabbit
2026-05-30 11:56:46.443463+01:00 [info] <0.270.0> Limiting to approx 927 file handles (832 sockets)
2026-05-30 11:56:46.443653+01:00 [info] <0.271.0> FHC read buffering: OFF
2026-05-30 11:56:46.443685+01:00 [info] <0.271.0> FHC write buffering: ON
2026-05-30 11:56:46.444453+01:00 [warning] <0.211.0> Available file handles: 1024. Please consider increasing system limits
2026-05-30 11:56:46.444491+01:00 [info] <0.211.0> Running boot step worker_pool defined by app rabbit
2026-05-30 11:56:46.444622+01:00 [info] <0.252.0> Will use 2 processes for default worker pool
2026-05-30 11:56:46.444658+01:00 [info] <0.252.0> Starting worker pool 'worker_pool' with 2 processes in it
2026-05-30 11:56:46.448537+01:00 [info] <0.211.0> Running boot step rabbit_registry defined by app rabbit
2026-05-30 11:56:46.448646+01:00 [info] <0.211.0> Running boot step database defined by app rabbit
2026-05-30 11:56:46.479034+01:00 [info] <0.211.0> Peer discovery: configured backend: rabbit_peer_discovery_classic_config
2026-05-30 11:56:46.479623+01:00 [notice] <0.211.0> Found the following metadata store members: [{rabbitmq_metadata,
2026-05-30 11:56:46.479623+01:00 [notice] <0.211.0> rabbit@localhost}]
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> Failed to get total system memory:
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {badmatch,{error,enoent}}
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{vm_memory_monitor,read_proc_file,1,
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,596}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory,1,
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,510}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory_from_os,0,
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,308}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {rabbit_disk_monitor,enable_handle_disk_free,2,
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{file,"rabbit_disk_monitor.erl"},{line,444}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {rabbit_disk_monitor,handle_info,2,
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> [{file,"rabbit_disk_monitor.erl"},{line,194}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {gen_server,try_handle_info,3,[{file,"gen_server.erl"},{line,2434}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {gen_server,handle_msg,3,[{file,"gen_server.erl"},{line,2420}]},
2026-05-30 11:58:46.445636+01:00 [warning] <0.267.0> {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,333}]}]
2026-05-30 11:58:46.446284+01:00 [warning] <0.267.0> Free disk space monitor encountered an error retrieving total memory. Retries left: 9 Error:
2026-05-30 11:58:46.446284+01:00 [warning] <0.267.0> unknown
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> Failed to get total system memory:
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {badmatch,{error,enoent}}
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{vm_memory_monitor,read_proc_file,1,
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,596}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory,1,
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,510}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {vm_memory_monitor,get_total_memory_from_os,0,
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{file,"vm_memory_monitor.erl"},{line,308}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {rabbit_disk_monitor,enable_handle_disk_free,2,
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{file,"rabbit_disk_monitor.erl"},{line,444}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {rabbit_disk_monitor,handle_info,2,
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> [{file,"rabbit_disk_monitor.erl"},{line,194}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {gen_server,try_handle_info,3,[{file,"gen_server.erl"},{line,2434}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {gen_server,handle_msg,3,[{file,"gen_server.erl"},{line,2420}]},
2026-05-30 12:00:46.452833+01:00 [warning] <0.267.0> {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,333}]}]
2026-05-30 12:00:46.453479+01:00 [warning] <0.267.0> Free disk space monitor encountered an error retrieving total memory. Retries left: 8 Error:
2026-05-30 12:00:46.453479+01:00 [warning] <0.267.0> unknown
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0>
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> BOOT FAILED
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> ===========
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> Exception during startup:
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0>
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> exit:{error,timeout}
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0>
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> rabbit_boot_steps:-run_step/2-lc$^0/1-0-/2, line 57
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> rabbit_boot_steps:run_step/2, line 60
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> rabbit_boot_steps:-run_boot_steps/1-lc$^0/1-0-/1, line 24
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> rabbit_boot_steps:run_boot_steps/1, line 25
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> rabbit:start/2, line 1013
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0> application_master:start_it_old/4, line 299
2026-05-30 12:01:46.481141+01:00 [error] <0.211.0>
2026-05-30 12:01:47.486245+01:00 [notice] <0.45.0> Application rabbit exited with reason: {{error,timeout},{rabbit,start,[normal,[]]}}
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> crasher:
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> initial call: application_master:init/3
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> pid: <0.210.0>
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> registered_name: []
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> exception exit: {{error,timeout},{rabbit,start,[normal,[]]}}
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> in function application_master:init/3 (application_master.erl:147)
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> ancestors: [application_controller,<0.10.0>]
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> message_queue_len: 1
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> messages: [{'EXIT',<0.211.0>,normal}]
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> links: [<0.45.0>]
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> dictionary: []
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> trap_exit: true
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> status: running
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> heap_size: 2586
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> stack_size: 29
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> reductions: 80
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0> neighbours:
2026-05-30 12:01:47.486546+01:00 [error] <0.210.0>
```
</details>
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