libzdb: data: avoid fragmentation of data files #158
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Data files are intended to be used on HDD when used on storage purpose, pre-allocating data space (without really write actual data) ensure that enough space is available and reduce fragmentation. If this call fails, allocation is just ignored and old behavior is used.
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Main idea of zdb was initially to be efficient on HDD, current implementation was doing always-append but per namespace, which means random write to different namespace at the same time would eventually produce fragmentation.
New implementation ensure data are (lazily) pre-allocated using
fallocateby reserving blocks when creating data file.