24. Store ``swap meta'' Description

``swap meta'' refers to a section of meta data stored at the beginning of an object that is stored on disk. This meta data includes information such as the object's cache key (MD5), URL, and part of the StoreEntry structure.

The meta data is stored using a TYPE-LENGTH-VALUE format. That is, each chunk of meta information consists of a TYPE identifier, a LENGTH field, and then the VALUE (which is LENGTH octets long).

24.1 Types

As of Squid-2.3, the following TYPES are defined (from enums.h):

STORE_META_VOID

Just a placeholder for the zeroth value. It is never used on disk.

STORE_META_KEY_URL

This represents the case when we use the URL as the cache key, as Squid-1.1 does. Currently we don't support using a URL as a cache key, so this is not used.

STORE_META_KEY_SHA

For a brief time we considered supporting SHA (secure hash algorithm) as a cache key. Nobody liked it, and this type is not currently used.

STORE_META_KEY_MD5

This represents the MD5 cache key that Squid currently uses. When Squid opens a disk file for reading, it can check that this MD5 matches the MD5 of the user's request. If not, then something went wrong and this is probably the wrong object.

STORE_META_URL

The object's URL. This also may be matched against a user's request for cache hits to make sure we got the right object.

STORE_META_STD

This is the ``standard metadata'' for an object. Really its just this middle chunk of the StoreEntry structure:

        time_t timestamp;
        time_t lastref;
        time_t expires;
        time_t lastmod;
        size_t swap_file_sz;
        u_short refcount;
        u_short flags;

STORE_META_HITMETERING

Reserved for future hit-metering (RFC 2227) stuff.

STORE_META_VALID

?

STORE_META_END

Marks the last valid META type.

24.2 Implementation Notes

When writing an object to disk, we must first write the meta data. This is done with a couple of functions. First, storeSwapMetaPack() takes a StoreEntry as a parameter and returns a tlv linked list. Second, storeSwapMetaPack() converts the tlv list into a character buffer that we can write.

Note that the MemObject has a member called swap_hdr_sz. This value is the size of that character buffer; the size of the swap file meta data. The StoreEntry has a member named swap_file_sz that represents the size of the disk file. Thus, the size of the object ``content'' is

        StoreEntry->swap_file_sz  - MemObject->swap_hdr_sz;

When reading a swap file, there is a similar process to extract the swap meta data. First, storeSwapMetaUnpack() converts a character buffer into a tlv linked list. It also tells us the value for MemObject->swap_hdr_sz.