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“The Cyclic Object Storage Scheme (costs) is an attempt to develop a custom file system for Squid.”

Ref: squid the definite guide Duane Wessels

Correction: per developer response

“Squid does not support COSS cache_dirs since v3.5. If Squid in question does disk caching, then rock cache_dirs may be the best bet.

Alex.”

Rock is not listed

====  The rock store type  ====

Usage:
        cache_dir rock Directory-Name Mbytes [options]

The Rock Store type is a database-style storage. All cached
    entries are stored in a "database" file, using fixed-size slots.
    A single entry occupies one or more slots.

If possible, Squid using Rock Store creates a dedicated kid
    process called "disker" to avoid blocking Squid worker(s) on disk
    I/O. One disker kid is created for each rock cache_dir.  Diskers
    are created only when Squid, running in daemon mode, has support
    for the IpcIo disk I/O module.

swap-timeout=msec: Squid will not start writing a miss to or
    reading a hit from disk if it estimates that the swap operation
    will take more than the specified number of milliseconds. By
    default and when set to zero, disables the disk I/O time limit
    enforcement. Ignored when using blocking I/O module because
    blocking synchronous I/O does not allow Squid to estimate the
    expected swap wait time.

max-swap-rate=swaps/sec: Artificially limits disk access using
    the specified I/O rate limit. Swap out requests that
    would cause the average I/O rate to exceed the limit are
    delayed. Individual swap in requests (i.e., hits or reads) are
    not delayed, but they do contribute to measured swap rate and
    since they are placed in the same FIFO queue as swap out
    requests, they may wait longer if max-swap-rate is smaller.
    This is necessary on file systems that buffer "too
    many" writes and then start blocking Squid and other processes
    while committing those writes to disk.  Usually used together
    with swap-timeout to avoid excessive delays and queue overflows
    when disk demand exceeds available disk "bandwidth". By default
    and when set to zero, disables the disk I/O rate limit
    enforcement. Currently supported by IpcIo module only.

slot-size=bytes: The size of a database "record" used for
    storing cached responses. A cached response occupies at least
    one slot and all database I/O is done using individual slots so
    increasing this parameter leads to more disk space waste while
    decreasing it leads to more disk I/O overheads. Should be a
    multiple of your operating system I/O page size. Defaults to
    16KBytes. A housekeeping header is stored with each slot and
    smaller slot-sizes will be rejected. The header is smaller than
    100 bytes.

==== COMMON OPTIONS ====

no-store    no new objects should be stored to this cache_dir.

min-size=n    the minimum object size in bytes this cache_dir
            will accept.  It's used to restrict a cache_dir
            to only store large objects (e.g. AUFS) while
            other stores are optimized for smaller objects
            (e.g. Rock).
            Defaults to 0.

max-size=n    the maximum object size in bytes this cache_dir
            supports.
            The value in maximum_object_size directive sets
            the default unless more specific details are
            available (ie a small store capacity).

Note: To make optimal use of the max-size limits you should order
    the cache_dir lines with the smallest max-size value first.

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