public-inbox-tuning - Man Page

If you're starting a new inbox (and not mirroring an existing one), the -V2 requires DBD::SQLite, but is orders of magnitude more scalable than the original -V1 format.

Our optional use of Inline::C speeds up subprocess spawning from large daemon processes.

To enable Inline::C, either set the PERL_INLINE_DIRECTORY environment variable to point to a writable directory, or create ~/.cache/public-inbox/inline-c for any user(s) running public-inbox processes.

If libgit2 development files are installed and Inline::C is enabled (described above), per-inbox git cat-file --batch processes are replaced with a single perl(1) process running PublicInbox::Gcf2::loop in read-only daemons.  libgit2 use will be available in public-inbox 1.7.0+

More (optional) Inline::C use will be introduced in the future to lower memory use and improve scalability.

Note: Inline::C is required for lei(1), but not public-inbox-*

Random I/O performance is poor on rotational HDDs.  Xapian indexing performance degrades significantly as DBs grow larger than available RAM.  Attempts to parallelize random I/O on HDDs leads to pathological slowdowns as inboxes grow.

While -V2 introduced Xapian shards as a parallelization mechanism for SSDs; enabling publicInbox.indexSequentialShard repurposes sharding as mechanism to reduce the kernel page cache footprint when indexing on HDDs.

Initializing a mirror with a high --jobs count to create more shards (in -V2 inboxes) will keep each shard smaller and reduce its kernel page cache footprint.  Keep in mind excessive sharding imposes a performance penalty for read-only queries.

Users with large amounts of RAM are advised to set a large value for publicinbox.indexBatchSize as documented in public-inbox-index(1).

dm-crypt users on Linux 4.0+ are advised to try the --perf-same_cpu_crypt --perf-submit_from_crypt_cpus switches of cryptsetup(8) to reduce I/O contention from kernel workqueue threads.

btrfs(5) performance degrades from fragmentation when using large databases and random writes.  The Xapian + SQLite indices used by public-inbox are no exception to that.

public-inbox 1.6.0+ disables copy-on-write (CoW) on Xapian and SQLite indices on btrfs to achieve acceptable performance (even on SSD). Disabling copy-on-write also disables checksumming, thus raid1 (or higher) configurations may be corrupt after unsafe shutdowns.

Fortunately, these SQLite and Xapian indices are designed to recoverable from git if missing.

Disabling CoW does not prevent all fragmentation.  Large values of publicInbox.indexBatchSize also limit fragmentation during the initial index.

Avoid snapshotting subvolumes containing Xapian and/or SQLite indices. Snapshots use CoW despite our efforts to disable it, resulting in fragmentation.

filefrag(8) can be used to monitor fragmentation, and btrfs filesystem defragment -fr $INBOX_DIR may be necessary.

Large filesystems benefit significantly from the space_cache=v2 mount option documented in btrfs(5).

Older, non-CoW filesystems are generally work well out-of-the-box for our Xapian and SQLite indices.

While SSD read performance is generally good, SSD write performance degrades as the drive ages and/or gets full.  Issuing TRIM commands via fstrim(8) or similar is required to sustain write performance.

Users of the Flash-Friendly File System F2FS <https://en.wikipedia.org/wiki/F2FS> may benefit from optimizations found in SQLite 3.21.0+.  Benchmarks are greatly appreciated.

public-inbox-httpd(1), public-inbox-imapd(1), and public-inbox-nntpd(1) are all designed for C10K (or higher) levels of concurrency from a single process.  SMP systems may use --worker-processes=NUM as documented in public-inbox-daemon(8) for parallelism.

The open file descriptor limit (RLIMIT_NOFILE, ulimit -n in sh(1), LimitNOFILE= in systemd.exec(5)) may need to be raised to accommodate many concurrent clients.

Transport Layer Security (IMAPS, NNTPS, or via STARTTLS) significantly increases memory use of client sockets, sure to account for that in capacity planning.

Linux users: the sys.vm.max_map_count sysctl may need to be increased if handling thousands of inboxes (with public-inbox-extindex(1)) to avoid out-of-memory errors from git.

Other OSes may have similar tuning knobs (patches appreciated).

public-inbox-extindex(1) allows any number of public-inboxes to share the same Xapian indices.

git 2.33+ startup time is orders-of-magnitude faster and uses less memory when dealing with thousands of alternates required for thousands of inboxes with public-inbox-extindex(1).

Frequent packing (via git-gc(1)) both improves performance and reduces the need to increase sys.vm.max_map_count.