nbdkit-blocksize-filter1

Usage

Overview

"nbdkit-blocksize-filter" is a filter that ensures various block size limits are met on transactions presented to the plugin. The NBD protocol permits clients to send requests with a granularity as small as 1 byte or as large as nearly 4 gigabytes, although it suggests that portable clients should align requests to 512 bytes and not exceed 32 megabytes without prior coordination with the server.

Meanwhile, some plugins require requests to be aligned to 512-byte multiples, or may enforce a maximum transaction size to bound the time or memory resources spent by any one command (note that nbdkit itself refuses a read or write larger than 64 megabytes, while many other NBD servers limit things to 32 megabytes). The blocksize filter can be used to modify the client requests to meet the plugin restrictions.

This filter can be combined with nbdkit-blocksize-policy-filter(1) to advertise different block sizes to the client.

Parameters

The nbdkit-blocksize-filter accepts the following parameters.

minblock=SIZE

The minimum block size and alignment to pass to the plugin. This must be a power of two, and no larger than 64k. If omitted, this defaults to the minimum block size of the underlying plugin, or 1 if the plugin did not report a minimum block size. The filter rounds up read requests to alignment boundaries, performs read-modify-write cycles for any unaligned head or tail of a write or zero request, and silently ignores any unaligned head or tail of a trim request. The filter also truncates the plugin size down to an aligned value (as it cannot safely operate on the unaligned tail). 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 parameter understands the suffix 'k' for 1024.

maxdata=SIZE

The maximum block size for any single transaction with data (read and write). If omitted, this defaults to the minimum of 64 megabytes (that is, the nbdkit maximum) or any maximum reported by the underlying plugin. This need not be a power of two, but must be an integer multiple of "minblock". The filter fragments any larger client request into multiple plugin requests.

This parameter understands the suffixes 'k', 'M', and 'G' for powers of 1024.

maxlen=SIZE

The maximum length for any single transaction without data (trim, zero, cache or extents). If omitted, this defaults to 0xffffffff rounded down to "minblock" alignment (that is, the inherent 32-bit limit of the NBD protocol). This need not be a power of two, but must be an integer multiple of "minblock", and should be at least as large as "maxdata". The filter fragments any larger client request into multiple plugin requests.

This parameter understands the suffixes 'k', 'M', and 'G' for powers of 1024.

Examples

Allow an arbitrary client to use the VDDK plugin (which is limited to 512-byte blocks), without having to fix the client to avoid sending unaligned requests:

 nbdkit --filter=blocksize vddk minblock=512 file=/absolute/path/to/file.vmdk

Allow an arbitrary client tuned to nbdkit's 64 megabyte sizing to connect to a remote server that insists on 32 megabyte sizing, via the nbd plugin:

 nbdkit --filter=blocksize nbd maxdata=32M socket=/path/to/socket

Serve a file as if it were a block device that insists on 4k alignment, while still allowing access to any unaligned bytes at the end of the file:

 nbdkit --filter=blocksize --filter=truncate file /path/to/file \
 minblock=4k round-up=4k

Files

$filterdir/nbdkit-blocksize-filter.so

The filter.

Use "nbdkit --dump-config" to find the location of $filterdir.

Version

"nbdkit-blocksize-filter" first appeared in nbdkit 1.4.