[QA] write to external SFTP storage is slow

setup: owncloud core 10.6.0

with

apt-get install pure-ftpd
adduser ftpdata

-> Settings -> Storage
SFTP, Username and password
Configuration
localhost /home/ftpdata

• A user moves a 200MB file from his primary storage to SFTP using the web UI.
  System loadavg is 0.01
  
• strace shows an sshd process with

read(4, "\2432\10\277\0056]\374\237\216\226\362\226w\261\237\24;jS\n\362\345]\21\t\10\253\226\322\36{"..., 16384) = 96
write(11, "\0\0\0(\3\0\0\0\1\0\0\0\27/home/ftpdata/200MB"..., 44) = 44
read(12, "\0\0\0\rf\0\0\0\1\0\0\0\4\0\0\0\0", 16384) = 17
write(4, "Y\247\216\36\16\370#\364%\331\10\0E\31\272\314R\317\220\277W\204\361.[\261\227z\376!\366g"..., 80) = 80
read(4, "\4S\3\320\377\\>\312F\0217\200>\226?\373CQ+!\330\341\351\376\350\353\22\16\303L\364\271"..., 16384) = 8192
read(4, "^\2605\257!R:@\311\364\331\362\1\353\335\221\335\237U\244\36\201\31\224\310c\244p\361\1gy"..., 16384) = 80
write(11, "\0\0 \31\6\0\0\0\0\0\0\0\4\0\0\0\0\0\0\0\0\4\332 \0\0\0 \0\266\206\346"..., 8221) = 8221
read(12, "\0\0\0\30e\0\0\0\0\0\0\0\0\0\0\0\7Success\0\0\0\0", 16384) = 28
write(4, "\235\300H\273\341\3314\342s\3\35LL\323\35\25R\337$\361\267\200\256m\247C\25\225\335\263\36\376"..., 80) = 80
read(4, "\346\370T\267\3514\314\245\335\206\241p\344\301\273\17\331F6\303\314\260hY\240\16\234\215\270\276:v"..., 16384) = 80
write(11, "\0\0\0\r\4\0\0\0\1\0\0\0\4\0\0\0\0", 17) = 17
read(12, "\0\0\0\30e\0\0\0\1\0\0\0\0\0\0\0\7Success\0\0\0\0", 16384) = 28
write(4, "\205\345\350z\325\362\372\212\230\21\0107\316\245\201\370\243\237\206Wy\332\333%0R\234 \354\270z\331"..., 80) = 80
read(4, "\252,)u\240\342\1776s0\303\264\275\263\240\311\273&g\t0\243_\24P!N\334\22\21(c"..., 16384) = 96
write(11, "\0\0\0(\3\0\0\0\1\0\0\0\27/home/ftpdata/200MB"..., 44) = 44
read(12, "\0\0\0\rf\0\0\0\1\0\0\0\4\0\0\0\0", 16384) = 17
write(4, "\372Eq\267\"\272A\215\35\377\0\21|\206.\316y\2\16\335O\224\3374\217\220z9\304\316\26l"..., 80) = 80
read(4, "0\333e\2l\311\6\232s\373T\272D4\315\326[A\23 \23\275\179\366\31,\330\2068\330\246"..., 16384) = 8192
read(4, "\207\304#\376\10.\37n\252\353v\22\2610V\233\36\272\320w\354~\213(G\377\213i}a\312\252"..., 16384) = 80
write(11, "\0\0 \31\6\0\0\0\0\0\0\0\4\0\0\0\0\0\0\0\0\4\332@\0\0\0 \0B\266\333"..., 8221) = 8221
read(12, "\0\0\0\30e\0\0\0\0\0\0\0\0\0\0\0\7Success\0\0\0\0", 16384) = 28
write(4, "F2\312l\354\177\323\303r\324\0375\2635\316(\24\0377\337\4\326\356\214\371\371)\222X<\vm"..., 80) = 80
read(4, "\270\4\324\252\327\375\310\341\r&\363@D\376\374\5\324wN6\321\336Z(\202\214I\245-\261\21X"..., 16384) = 80
write(11, "\0\0\0\r\4\0\0\0\1\0\0\0\4\0\0\0\0", 17) = 17
read(12, "\0\0\0\30e\0\0\0\1\0\0\0\0\0\0\0\7Success\0\0\0\0", 16384) = 28
write(4, "~\342\301l4\225U?\222%&\240\231\1]\276'(g\266 \22X\246\36\216ZX\342Y\345\0"..., 80) = 80
read(4, "G\363v\222\354\321#\327`S\324d\354\360\252n5/\261\2202\321]P\2S\377\341\26\220\277\361"..., 16384) = 96
write(11, "\0\0\0(\3\0\0\0\1\0\0\0\27/home/ftpdata/200MB"..., 44) = 44
read(12, "\0\0\0\rf\0\0\0\1\0\0\0\4\0\0\0\0", 16384) = 17
write(4, "\222\251\32\377[\3412\235\334\363B\16E`te\tF\260}\303\267\220v\26V\375\203$\204\360\207"..., 80) = 80

80 byte read write system calls are inefficient. BAD.

• the move finishes after 22 minutes. BAD

Expected behaviour: Do 8192 byte chunks more often. loadavg ca 1.0 and a few seconds max. (A manual sftp "put" into that storage reaches > 100MBytes / sec)

Start from the web UI flow for moving a 200MB file to configured SFTP storage and trace the external-storage write path. Reproduce with the provided pure-ftpd setup, compare strace output with a manual SFTP put, and verify that the transfer uses larger, repeated chunks and completes in seconds rather than 22 minutes.