Bicycle chain

Jovan Stosic July 4, 2020

Width

Chains come in 3⁄32 in (2.4 mm), 1⁄8 in (3.2 mm), 5⁄32 in (4.0 mm), or 3⁄16 in (4.8 mm) roller widths, the internal width between the inner plates. 1⁄8 in (3.2 mm) chains are typically used on bikes with a single rear sprocket: those with coaster brakes, hub gears, fixed gears such as track bicycles, or BMX bikes. Chains with 3⁄32 in (2.4 mm) wide rollers are generally used on bikes with derailleurs such as racing, touring, and mountain bikes.[17] Fixed sprockets and freewheels are also available in 3⁄32 in (2.4 mm) widths so fixed-gear and single-speed bikes can be set up to use the narrower and lighter 3⁄32 in (2.4 mm) chains. Finally, chains with 5⁄32 in (4.0 mm) wide rollers are used on freight bicycles and tricycles.

With derailleur equipped bicycles, the external width of the chain (measured at the connecting rivet) also matters, because chains must not be too wide for the cogset or they will rub on the next larger sprocket, or too narrow that they might fall between two sprockets. Chains can also be identified by the number of rear sprockets they can support, anywhere from 3 to 12, and the list below enables measuring a chain of unknown origin to determine its suitability.

  • 6 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
  • 7 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
  • 8 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
  • 9 speed – 6.5 to 7.0 mm (1⁄4 to 9⁄32 in) (all brands)
  • 10 speed – 6.0 to 7.0 mm (1⁄4 to 9⁄32 in) (Shimano, Campagnolo)
  • 10 speed (Narrow) – 5.88 mm (7⁄32 in) (Campagnolo, KMC)
  • 10 speed (Narrow, Direction) – 5.88 mm (7⁄32 in) (Shimano CN-5700, CN-6700, CN-7900)
  • 11 speed – 5.5 to 5.62 mm (7⁄32 to 7⁄32 in) (Campagnolo, KMC, Shimano CN-9000)
  • 12 speed – 5.3 mm (13⁄64 in) (SRAM)

The Wikibook, “Bicycle Maintenance and Repair”, has more details on this topic. Shimano uses the same chain types on 6, 7,and 8 speed designs.

https://en.wikipedia.org/wiki/Bicycle_chain

WritingRules in SPAMASSASSIN

Jovan Stosic June 30, 2020

For our first rule, let’s start with the simplest type of rules, the basic “body” rule. These rules search the body of the message with a regular expression and if it matches, the corresponding score is assigned. Body rules also include the Subject as the first line of the body content. See [DumpTextPlugin]
Let’s look at a really basic fictitious rule:

body LOCAL_DEMONSTRATION_RULE	/test/
score LOCAL_DEMONSTRATION_RULE 0.1
describe LOCAL_DEMONSTRATION_RULE 	This is a simple test rule

This rule does a simple case-sensitive search of the body of the email for the string “test” and adds a 0.1 to the score of the email if it finds it. Now, this rule is pretty simple as rules go. It will match “test” but also “testing” and “attest”. The describe statement contains the text which will be placed into the verbose report, if verbose reports are used (this is the default setting for the body, in Spamassassin version 2.5x and upwards).

In regular expressions a \b can be used to indicate where a word-break (anything that isn’t an alphanumeric character or underscore) must exist for a match. Our rule above can be made to not match “testing” or “attest” like so:

body LOCAL_DEMONSTRATION_RULE	/\btest\b/

The rule can also be made case-insensitive by adding an i to the end, like this:

body LOCAL_DEMONSTRATION_RULE	/\btest\b/i
score LOCAL_DEMONSTRATION_RULE 0.1

Now the rule will match any combination of upper or lower case that spells “test” surrounded by word breaks of some form.

Source: WritingRules – SPAMASSASSIN – Apache Software Foundation

NFS – Dovecot Wiki

Jovan Stosic June 29, 2020

NFS

NFS is commonly used in one of these ways:

  1. Dovecot is run in a single computer.
  2. Dovecot is run in multiple computers, users are redirected more or less randomly to different computers.
  3. Dovecot is run in multiple computers, each user is assigned a specific computer which is used whenever possible.

The only way to reliably implement the 2nd setup is with the director service.

Dovecot configuration

Single Dovecot server setup or Dovecot director cluster setup:

mmap_disable = yes
#dotlock_use_excl = no # only needed with NFSv2, NFSv3+ supports O_EXCL and it's faster
mail_fsync = always
mail_nfs_storage = no
mail_nfs_index = no

Multi-server setup that tries to flush NFS caches (increases NFS operations, and isn’t fully reliable), try not to use this:

mmap_disable = yes
#dotlock_use_excl = no # only needed with NFSv2, NFSv3+ supports O_EXCL and it's faster
mail_fsync = always
# These settings slow things down and don't fully work, use director proxy instead:
mail_nfs_storage = yes
mail_nfs_index = yes

Common issues

Clock synchronization

Run ntpd in the NFS server and all the NFS clients to make sure their clocks are synchronized. If the clocks are more than one second apart from each others and multiple computers access the same mailbox simultaneously, you may get errors.

NFS caching problems

NFS caching is a big problem when multiple computers are accessing the same mailbox simultaneously. The best fix for this is to prevent it from happening. Configure your setup so that a user always gets redirected to the same server (unless it’s down). This also means that mail deliveries must be done by the same server, or alternatively it shouldn’t update index files.

Dovecot flushes NFS caches when needed if you set mail_nfs_storage=yes, but unfortunately this doesn’t work 100%, so you can get random errors.

Disabling NFS attribute cache helps a lot in getting rid of caching related errors, but this makes the performance MUCH worse and increases the load on NFS server. This can usually be done by giving actimeo=0 or noac mount option.

Index files

If you keep the index files stored on NFS, you’ll need to set mmap_disable=yes. If you’re not running lockd you’ll have to set lock_method=dotlock, but this degrades performance. Note that some NFS installations have problems with lockd. If you’re beginning to get all kinds of locking related errors, try if the problems go away with dotlocking.

With mbox/Maildir formats (but not dbox!) it’s also possible to store index files on local disk instead of on NFS. If the user gets redirected to different servers, the local indexes are automatically created/updated. If the user is (nearly) always redirected to the same server this should be fine and you would likely get higher performance than indexes stored on NFS, but if the server changes it can be slow to recreate the index/cache files.

Source: NFS – Dovecot Wiki