Storage backends¶
Intro¶
Varnish has pluggable storage backends. It can store data in various backends which can have different performance characteristics. The default configuration is to use the malloc backend with a limited size. For a serious Varnish deployment you probably would want to adjust the storage settings.
malloc¶
syntax: malloc[,size]
Malloc is a memory based backend. Each object will be allocated from memory. If your system runs low on memory swap will be used.
Be aware that the size limitation only limits the actual storage and that the approximately 1k of memory per object, used for various internal structures, is included in the actual storage as well.
The size parameter specifies the maximum amount of memory varnishd will allocate. The size is assumed to be in bytes, unless followed by one of the following suffixes:
K, k The size is expressed in kibibytes.
M, m The size is expressed in mebibytes.
G, g The size is expressed in gibibytes.
T, t The size is expressed in tebibytes.
The default size is unlimited.
malloc’s performance is bound to memory speed so it is very fast. If the dataset is bigger than available memory performance will depend on the operating systems ability to page effectively.
file¶
syntax: file,path[,size[,granularity[,advice]]]
The file backend stores objects in memory backed by an unlinked file on disk with mmap.
The ‘path’ parameter specifies either the path to the backing file or the path to a directory in which varnishd will create the backing file.
The size parameter specifies the size of the backing file. The size is assumed to be in bytes, unless followed by one of the following suffixes:
K, k The size is expressed in kibibytes.
M, m The size is expressed in mebibytes.
G, g The size is expressed in gibibytes.
T, t The size is expressed in tebibytes.
If ‘path’ points to an existing file and no size is specified, the size of the existing file will be used. If ‘path’ does not point to an existing file it is an error to not specify the size.
If the backing file already exists, it will be truncated or expanded to the specified size.
Note that if varnishd has to create or expand the file, it will not pre-allocate the added space, leading to fragmentation, which may adversely impact performance on rotating hard drives. Pre-creating the storage file using dd(1) will reduce fragmentation to a minimum.
The ‘granularity’ parameter specifies the granularity of allocation. All allocations are rounded up to this size. The granularity is is assumed to be expressed in bytes, unless followed by one of the suffixes described for size.
The default granularity is the VM page size. The size should be reduced if you have many small objects.
File performance is typically limited to the write speed of the device, and depending on use, the seek time.
‘advice’ dictates what Varnish tells the system to optimize reads. Depending on your OS, disks and object sizes, it can be beneficial to tweak this. The three possible values are “normal”, “random” (default) and “sequential” and correspond to MADV_NORMAL, MADV_RANDOM, MADV_SEQUENTIAL, respectively. For example, large objects and rotational disk should profit from “sequential” on Linux.
persistent (experimental)¶
syntax: persistent,path,size {experimental}
Persistent storage. Varnish will store objects in a file in a manner that will secure the survival of most of the objects in the event of a planned or unplanned shutdown of Varnish.
The ‘path’ parameter specifies the path to the backing file. If the file doesn’t exist Varnish will create it.
The ‘size’ parameter specifies the size of the backing file. The size is expressed in bytes, unless followed by one of the following suffixes:
K, k The size is expressed in kibibytes.
M, m The size is expressed in mebibytes.
G, g The size is expressed in gibibytes.
T, t The size is expressed in tebibytes.
Varnish will split the file into logical silos and write to the silos in the manner of a circular buffer. Only one silo will be kept open at any given point in time. Full silos are sealed. When Varnish starts after a shutdown it will discard the content of any silo that isn’t sealed.
Note that taking persistent silos offline and at the same time using bans can cause problems. This is due to the fact that bans added while the silo was offline will not be applied to the silo when it reenters the cache. Consequently enabling previously banned objects to reappear.
Transient Storage¶
If you name any of your storage backend “Transient” it will be used for transient (short lived) objects. This includes the temporary objects created when returning a synthetic object. By default Varnish would use an unlimited malloc backend for this.
Varnish will consider an object short lived if the TTL is below the parameter ‘shortlived’.