slow performance due to small buffer used in mc_read()

[ioan]

hi

mc_read() in file.c uses 8K buffer while copying files. i've maked some measurements of copying speed under two machines:
1) mac os, 10gbit network. copying to NAS (which have shared SSD with >1Gbyte/s throughput) using NFS - 150mb/s
2) ubuntu, local copying between two virtual disks - 350mb/s.

now i modify buffer 'buf', which used by mc_read()/wc_write() as temporary buffer for copying. i increased it's size from 8kbytes to 2mbytes. how we have following situation:
1) under mac os, speed increased from 150mb/s to 385mb/s
2) under ubuntu, speed increased from 350mb/s to 1900mb/s.

i've tested various buf sizes and 2mbytes is optimal in my situation (i've tested mostly using NFS over 10gbit network).

[andrew_b]

Closed as duplicate of #2193.