nbdkit --filter=cache plugin [cache=writeback|writethrough|unsafe] [cache-max-size=SIZE] [cache-high-threshold=N] [cache-low-threshold=N] [cache-on-read=true|false] [plugin-args...]
nbdkit-cache-filter is a filter that adds caching on top of a plugin. This is useful if a plugin is slow or expensive to use, because nbdkit will try to minimize requests to the plugin by caching previous requests.
Note that many NBD clients are able to do caching, and because the caching happens on the client side it will usually be more effective than caching inside the server. This filter can be used if the client does not have effective caching, or (with
cache=unsafe) to defeat flush requests from the client (which is unsafe and can cause data loss, as the name suggests).
Note that the use of this filter rounds the image size down to a multiple of the caching granularity (the larger of 4096 or the
f_bsize field of fstatvfs(3)), to ease the implementation. If you need to round the image size up instead to access the last few bytes, combine this filter with nbdkit-truncate-filter(1).
This filter only caches image contents. To cache image metadata, use nbdkit-cacheextents-filter(1) between this filter and the plugin. To accelerate sequential reads, use nbdkit-readahead-filter(1) instead.
Store writes in the cache. They are not written to the plugin unless an explicit flush is done by the client.
This is the default caching mode, and is safe if your client issues flush requests correctly (which is true for modern Linux and other well-written NBD clients).
Always force writes through to the plugin.
This makes the cache less effective, but is necessary if your client does not issue correct flush requests.
Ignore flush requests. Never write to the plugin unless the cache grows too large.
This is dangerous and can cause data loss, but this may be acceptable if you only use it for testing or with data that you don't care about or can cheaply reconstruct.
Limit the size of the cache to
SIZE. See “Cache Maximum Size” below.
Cache read requests as well as write and cache requests. Any time a block is read from the plugin, it is saved in the cache (if there is sufficient space) so the same data can be served more quickly later.
Note that if the underlying data served by the plugin can be modified by some other means (eg. something else can write to a file which is being served by nbdkit-file-plugin(1)), this option will cause nbdkit to serve stale data because reads won't always go through to the plugin.
Do not cache read requests (this is the default).
Cache Maximum Size
By default the cache can grow to any size (although not larger than the virtual size of the underlying plugin) and you have to ensure there is sufficient space in
$TMPDIR for it.
Using the parameters
cache-low-threshold you can limit the maximum size of the cache.
This requires kernel and filesystem support (for fallocate(2)
FALLOC_FL_PUNCH_HOLE), so it may not work on all platforms.
The cache is limited to around 1 gigabyte.
- cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80
Once the cache size reaches 972M (95% of 1G), blocks are reclaimed from the cache until the size is reduced to 819M (80% of 1G).
The way this works is once the size of the cache exceeds
SIZE ✕ the high threshold, the filter works to reduce the size of the cache until it is less than
SIZE ✕ the low threshold. Once the size is below the low threshold, no more reclaim work is done until the size exceeds the high threshold again.
The default thresholds are high 95% and low 80%. You must set 0 < low < high. The thresholds are expressed as integer percentages of
Least recently used blocks are discarded first.
The cache is stored in a temporary file located in /var/tmp by default. You can override this location by setting the
TMPDIRenvironment variable before starting nbdkit.
nbdkit --dump-configto find the location of
nbdkit-cache-filter first appeared in nbdkit 1.2.
nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1), nbdkit-readahead-filter(1), nbdkit-truncate-filter(1), nbdkit-filter(3), qemu-img(1).
Richard W.M. Jones
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nbdkit(1), nbdkit-cacheextents-filter(1), nbdkit-filter(3), nbdkit-nocache-filter(1), nbdkit-readahead-filter(1), nbdkit-release-notes-1.10(1), nbdkit-release-notes-1.14(1), nbdkit-truncate-filter(1).