8.1-STABLE: zfs and sendfile: problem still exists

[Alexander Zagrebin]

Hi!

I've noticed that ZFS on 8.1-STABLE still has problems with sendfile.
When accessing a file at first time the transfer speed is too low, but
on following attempts the transfer speed is normal.

How to repeat:

$ dd if=/dev/random of=/tmp/test bs=1m count=100
100+0 records in
100+0 records out
104857600 bytes transferred in 5.933945 secs (17670807 bytes/sec)
$ sudo env LC_ALL=C /usr/libexec/ftpd -D

The first attempt to fetch file:

$ fetch -o /dev/null ftp://localhost/tmp/test
/dev/null                                       1% of  100 MB  118 kBps
14m07s^C
fetch: transfer interrupted

The transfer rate is too low (approx. 120 kBps), but any subsequent attempts
are success:

$ fetch -o /dev/null ftp://localhost/tmp/test
/dev/null                                     100% of  100 MB   42 MBps
$ fetch -o /dev/null ftp://localhost/tmp/test
/dev/null                                     100% of  100 MB   47 MBps
...

To repeat it is enough to copy a file and to try again:

$ cp /tmp/test /tmp/test1
$ fetch -o /dev/null ftp://localhost/tmp/test1
/dev/null                                       2% of  100 MB  119 kBps
13m50s^C
fetch: transfer interrupted
$ fetch -o /dev/null ftp://localhost/tmp/test1
/dev/null                                     100% of  100 MB   41 MBps
$ fetch -o /dev/null ftp://localhost/tmp/test1
/dev/null                                     100% of  100 MB   47 MBps

...and again:

$ cp /tmp/test1 /tmp/test2
$ fetch -o /dev/null ftp://localhost/tmp/test2
/dev/null                                       1% of  100 MB  118 kBps
14m07s^C
fetch: transfer interrupted
$ fetch -o /dev/null ftp://localhost/tmp/test2
/dev/null                                     100% of  100 MB   41 MBps
$ fetch -o /dev/null ftp://localhost/tmp/test2
/dev/null                                     100% of  100 MB   47 MBps

I've tried ftpd and nginx with "sendfile on". The behavior is the same.
After disabling using sendfile in nginx ("sendfile off") the problem has
gone.

-- 
Alexander Zagrebin