pgsql: Avoid fragmentation of logical tapes when writing concurrently.

Avoid fragmentation of logical tapes when writing concurrently.

Disk-based HashAgg relies on writing to multiple tapes
concurrently. Avoid fragmentation of the tapes' blocks by
preallocating many blocks for a tape at once. No file operations are
performed during preallocation; only the block numbers are reserved.

Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/20200519151202.u2p2gpiawoaznsv2%40development

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/896ddf9b3cd7dcf70e43f733ae8fec5dfe6e31af

Modified Files
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src/backend/utils/sort/logtape.c | 80 ++++++++++++++++++++++++++++++++++++++--
1 file changed, 77 insertions(+), 3 deletions(-)