Production postmortemThe big server that couldn’t handle the load
A user of ours called us, quite frantic. They are running a lot of systems on RavenDB, and have been for quite some time.
However, very recently they started to run into severe issues. RavenDB would complain that there isn’t sufficient memory to run.
The system metrics, however, said that there are still gobs of GBs available (I believe that this is the appropriate technical term).
After verifying the situation, the on-call engineer escalated the issue. The problem was weird. There was enough memory, for sure, but for some reason RavenDB would be unable to run properly.
An important aspect is that this user is running a multi-tenant system, with each tenant being served by its own database. Each database has a few indexes as well.
Once we figured that out, it was actually easy to understand what is going on.
There are actually quite a few limits that you have to take into account. I talked about them here. In that post, the issue was the maximum number of tasks defined by the system. After which, you can no longer create new threads.
In this case, the suspect was: vm.max_map_count.
Beyond just total memory, Linux has a limit on the number of memory mappings that a process may have. And RavenDB uses Voron, which is based on mmap(), and each database and each index typically have multiple maps going on.
Given the number of databases involved…
The solution was to increase the max_map_count and add a task for us, to give a warning to the user ahead of time when they are approaching the system's limits.
More posts in "Production postmortem" series:
- (27 Jan 2023) The server ate all my memory
- (23 Jan 2023) The big server that couldn’t handle the load
- (16 Jan 2023) The heisenbug server
- (03 Oct 2022) Do you trust this server?
- (15 Sep 2022) The missed indexing reference
- (05 Aug 2022) The allocating query
- (22 Jul 2022) Efficiency all the way to Out of Memory error
- (18 Jul 2022) Broken networks and compressed streams
- (13 Jul 2022) Your math is wrong, recursion doesn’t work this way
- (12 Jul 2022) The data corruption in the node.js stack
- (11 Jul 2022) Out of memory on a clear sky
- (29 Apr 2022) Deduplicating replication speed
- (25 Apr 2022) The network latency and the I/O spikes
- (22 Apr 2022) The encrypted database that was too big to replicate
- (20 Apr 2022) Misleading security and other production snafus
- (03 Jan 2022) An error on the first act will lead to data corruption on the second act…
- (13 Dec 2021) The memory leak that only happened on Linux
- (17 Sep 2021) The Guinness record for page faults & high CPU
- (07 Jan 2021) The file system limitation
- (23 Mar 2020) high CPU when there is little work to be done
- (21 Feb 2020) The self signed certificate that couldn’t
- (31 Jan 2020) The slow slowdown of large systems
- (07 Jun 2019) Printer out of paper and the RavenDB hang
- (18 Feb 2019) This data corruption bug requires 3 simultaneous race conditions
- (25 Dec 2018) Handled errors and the curse of recursive error handling
- (23 Nov 2018) The ARM is killing me
- (22 Feb 2018) The unavailable Linux server
- (06 Dec 2017) data corruption, a view from INSIDE the sausage
- (01 Dec 2017) The random high CPU
- (07 Aug 2017) 30% boost with a single line change
- (04 Aug 2017) The case of 99.99% percentile
- (02 Aug 2017) The lightly loaded trashing server
- (23 Aug 2016) The insidious cost of managed memory
- (05 Feb 2016) A null reference in our abstraction
- (27 Jan 2016) The Razor Suicide
- (13 Nov 2015) The case of the “it is slow on that machine (only)”
- (21 Oct 2015) The case of the slow index rebuild
- (22 Sep 2015) The case of the Unicode Poo
- (03 Sep 2015) The industry at large
- (01 Sep 2015) The case of the lying configuration file
- (31 Aug 2015) The case of the memory eater and high load
- (14 Aug 2015) The case of the man in the middle
- (05 Aug 2015) Reading the errors
- (29 Jul 2015) The evil licensing code
- (23 Jul 2015) The case of the native memory leak
- (16 Jul 2015) The case of the intransigent new database
- (13 Jul 2015) The case of the hung over server
- (09 Jul 2015) The case of the infected cluster