13.6 swap.state

A swap.state file is a journal of objects that have been added to, and removed from, a cache directory. Each cache_dir has its own swap.state file. When Squid starts up, it reads the swap.state files to rebuild its in-memory indexes of cached objects. These files are a relatively critical part of Squid's operation.

By default, each swap.state file is located in its corresponding cache directory. Thus, each state file automatically stays with each cache_dir. This is important if you ever decide to reorder your cache_dir lines or if you remove one or more from the list.

If you prefer to put them in a different location, you can use the cache_swap_log directive:

cache_swap_log /usr/local/squid/var/logs/swap.state

In this case, Squid creates a swap.state file for each directory by appending a numeric suffix. For example, if you have four cache directories, Squid creates the following:

/usr/local/squid/var/logs/swap.state.00

/usr/local/squid/var/logs/swap.state.01

/usr/local/squid/var/logs/swap.state.02

/usr/local/squid/var/logs/swap.state.03

In this situation, if you add, remove, or rearrange cache_dir lines, you may need to rename the swap.state files manually to keep everything consistent.

Technically, the swap.state format is storage scheme-dependent. However, all storage schemes use the same format in the current versions of Squid. The swap.state file uses a fixed-size (48-byte) binary format. Fields are written in host-byte order and are thus not necessarily portable between different operating systems. Table 13-3 describes the fields of a swap.state entry.

Table 13-3. swap.state entry fields
Name	Size, in bytes	Description
op	1	Operation on the entry: added (1) or deleted (2).
file number	4	Same as the fourth field of store.log, except it is stored in binary.
timestamp	4	A timestamp corresponding to the time when the response was generated or last validated. Taken from the `Date` header for responses that have one. Stored as the number of seconds since the Unix epoch.
lastref	4	A timestamp corresponding to the most recent access to the object.
expires	4	The object's expiration time, taken from an `Expires` header or `Cache-Control` `max-age` directive.
last-modified	4	The object's `Last-Modified` value.
swap file size	4	The amount of space the object occupies on disk. This includes HTTP headers and other Squid-specific meta-information.
refcount	2	The number of times this object has been requested.
flags	2	Various internal flags used by Squid.
key	16	The MD5 hash of the corresponding URI. Same as the key in store.log, except this one is stored in binary.