docs: add hardware sizing section and remove stale MySQL variables

Refs #215

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

Author: somethingwithproof

Requirements.md

@@ -1,5 +1,36 @@
 # Requirements
 
+## Hardware Sizing
+
+The table below covers common deployment sizes. These are starting points; actual
+requirements depend on polling interval, number of data sources per device, use of
+Spine vs cmd.php, and whether remote pollers are used.
+
+| Deployment size | Devices | Data sources | CPU cores | RAM | Disk |
+|---|---|---|---|---|---|
+| Small | < 500 | < 50,000 | 2 | 4 GB | 50 GB SSD |
+| Medium | 500–2,000 | 50,000–500,000 | 4–8 | 16 GB | 200 GB SSD |
+| Large | 2,000–10,000 | 500,000–2,000,000 | 8–16 | 32–64 GB | 1 TB NVMe |
+| Very large | > 10,000 | > 2,000,000 | 16+ | 64+ GB | 2+ TB NVMe |
+
+**Notes:**
+
+- Use SSD or NVMe for the RRD file directory. RRDtool performs many small random
+  writes; spinning disk causes polling backlogs on medium and larger installs.
+- The MySQL/MariaDB data directory benefits from SSD as well.
+  `innodb_doublewrite` protects against torn page writes during a crash;
+  disabling it reduces crash safety on any storage type, including SSD/NVMe.
+  Setting `innodb_doublewrite = OFF` is an advanced optimization; only
+  consider it when you accept the durability trade-off (e.g. strong
+  backup/replication strategy and tolerance for potential data loss on an
+  unclean shutdown).
+- Spine is CPU-bound. Each spine process spawns threads up to your configured
+  maximum; allocate 1–2 spine processes per CPU core for best throughput.
+- Very large deployments (> 10,000 devices) require remote pollers deployed close to the devices
+  they poll rather than scaling a single main poller vertically.
+
+## Software Requirements
+
 Cacti requires that the following software is installed on your system.
 
 > **Note**: As of Cacti 1.2.31, PHP 8.1 is required and PHP Composer is required. 
@@ -12,7 +43,7 @@ Cacti requires that the following software is installed on your system.
 
 - RRDtool 1.3 or greater, 1.5+ recommended
 
-- PHP 5.4 or greater, 5.5+ recommended
+- PHP 8.1 or greater
   - Required modules:
     - ctype, date, filter, gettext, gd, gmp, intl
     - hash, json, ldap, mbstring, openssl, pcre
@@ -30,17 +61,17 @@ Cacti requires that the following software is installed on your system.
 
     SELinux and ModSecurity can cause problem with ICMP ping, socket connections, ...
 
-- MySQL 5.6 or MariaDB 5.5 or greater
+- MySQL 5.7 or MariaDB 10.2 or greater
   - Timezone support must be enabled
 
   - The following are my.cnf recommendations:
 
-    - **version >= 5.6**
+    - **version >= 5.7**
 
-      MySQL 5.6+ and MariaDB 10.0+ are great releases, and are very good
-      versions to choose. Make sure you run the very latest release though
-      which fixes a long standing low level networking issue that was causing
-      spine many issues with reliability.
+      MySQL 5.7+ and MariaDB 10.2+ are supported. MySQL 8.0 and MariaDB 10.6+
+      are recommended for new installs. Run the latest maintenance release for
+      your chosen branch; older patch releases have known networking issues that
+      cause intermittent Spine failures.
 
     - **innodb = ON**
 
@@ -146,13 +177,12 @@ Cacti requires that the following software is installed on your system.
 
     - **innodb_doublewrite = OFF**
 
-      With modern SSD type storage, this operation actually degrades the disk
-      more rapidly and adds a 50% overhead on all write operations.
-
-    - **innodb_additional_mem_pool_size >= 80M**
-
-      This is where metadata is stored. If you had a lot of tables, it would
-      be useful to increase this.
+      Disables the InnoDB doublewrite buffer. This eliminates a 50% write
+      overhead on SSD/NVMe storage, but removes protection against partial page
+      writes on an unclean shutdown. Only set this on SSD or NVMe storage with
+      battery-backed or capacitor-backed write cache, or when the database is
+      on a volume with hardware-level write atomicity guarantees. Do not set on
+      spinning disk or consumer SSDs without power-loss protection.
 
     - **innodb_lock_wait_timeout >= 50**
 
@@ -225,9 +255,7 @@ paste them into my.cnf
  innodb_buffer_pool_size = 250M
  innodb_io_capacity = 5000
  innodb_io_capacity_max = 10000
- innodb_file_format = Barracuda
- innodb_large_prefix = 1
- ```
+```
 
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