thr3ads.net - rsync - Transfer hangs, both sides waiting on receive [Apr 2017]

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Apollon Oikonomopoulos
2017-Apr-05 11:49 UTC
Transfer hangs, both sides waiting on receive

Hi,

We're encountering a rather weird, sporadic hang during a large-file 
transfer. A bit of background on the setup:

 - Host A rsyncs a MySQL database from Host B. Both sides use rsync 
   3.1.1 on Debian Jessie.

 - Host B serves the files using rsync --daemon from an ext4 filesystem 
   residing on a thin LVM snapshot.

 - Host A uses the following command to rsync the files to a btrfs subvolume:
   rsync -og -avPH --log-file=/var/log/mysql-rsync.log --debug=ALL
--chown=user:user --delete --inplace rsync://hostb.example.com/mysql/
/media/btrfs/db-snapshot-$$/mysql
   (I know parts of the command (-og + -a + --chown) don't make much 
   sense :)

 - Both hosts sit on the same LAN, connected to a Juniper EX4200 stack 
   with active-backup interfaces. MTU is standard 1500 and there are no 
	 known network issues. Host A has Intel I350 (igb) NICs, while Host B has
   BCM5709 (bnx2) NICs. Both hosts run Linux 3.16.36.

Once every few (~3-5) transfers, the rsync process hangs with both sides 
waiting on select() on the TCP socket. From the transfer logs I can see 
that the hang always happens when about to receive the same 32GB file, 
already a couple of GBs into the total transfer.

Host A reports:
....
04:41:03 [6281] [receiver] send_msg_int(100, 1701)
04:41:03 [6281] recv_files(mysql/innodb_table_stats.ibd)
04:41:03 [6281] sender finished //mysql/innodb_table_stats.ibd
04:41:03 [6281] send_files(1749, //mysql/user.MYD)
04:41:03 [6281] sender finished //mysql/user.MYD
04:41:03 [6281] send_files(1750, //mysql/user.MYI)
04:41:03 [6281] sender finished //mysql/user.MYI
04:41:03 [6281] send_files(1806, //some_dbase)
04:41:03 [6281] send_files(1808, //some_dbase/actions.ibd)
04:41:03 [6281] >f.st...... mysql/innodb_table_stats.ibd
04:41:03 [6281] finishing mysql/innodb_table_stats.ibd
04:41:03 [6281] set modtime of mysql/innodb_table_stats.ibd to (1491343251) Wed
Apr  5 01:00:51 2017
04:41:03 [6281] [receiver] send_msg_int(100, 1703)
04:41:03 [6281] recv_files(mysql/user.MYD)
04:41:03 [6281] >f.st...... mysql/user.MYD
04:41:03 [6281] finishing mysql/user.MYD
04:41:03 [6281] set modtime of mysql/user.MYD to (1487686529) Tue Feb 21
16:15:29 2017
04:41:03 [6281] [receiver] send_msg_int(100, 1749)
04:41:03 [6281] recv_files(mysql/user.MYI)
04:41:03 [6281] >f..t...... mysql/user.MYI
04:41:03 [6281] finishing mysql/user.MYI
04:41:03 [6281] set modtime of mysql/user.MYI to (1487689840) Tue Feb 21
17:10:40 2017
04:41:03 [6281] [receiver] send_msg_int(100, 1750)
04:41:03 [6281] recv_files(some_dbase)
04:41:03 [6281] .d..t...... some_dbase/
<silence>

while Host B reports:
....
01:41:03 [3726] send_files(1749, //mysql/user.MYD)
01:41:03 [3726] <f.st...... mysql/user.MYD
01:41:03 [3726] sender finished //mysql/user.MYD
01:41:03 [3726] send_files(1750, //mysql/user.MYI)
01:41:03 [3726] <f..t...... mysql/user.MYI
01:41:03 [3726] sender finished //mysql/user.MYI
01:41:03 [3726] send_files(1806, //some_dbase)
01:41:03 [3726] .d..t...... some_dbase/
01:41:03 [3726] send_files(1808, //some_dbase/actions.ibd)
<silence>

The backtraces of the hung processes are:

Host A (receiver):
 #0  0x00007f87d7da3873 in select () from /lib/x86_64-linux-gnu/libc.so.6
 #1  0x00007f87d88dd7a2 in perform_io (needed=needed at entry=4, flags=flags at
entry=17) at io.c:742
 #2  0x00007f87d88deef7 in raw_read_int () at io.c:908
 #3  0x00007f87d88df683 in read_a_msg () at io.c:1423
 #4  0x00007f87d88e0676 in read_buf (f=f at entry=4, buf=buf at
entry=0x7ffd814c0df0 "\300\037L\201\375\177", len=len at entry=1) at
io.c:1835
 #5  0x00007f87d88e157b in read_ndx (f=f at entry=4) at io.c:2222
 #6  0x00007f87d88be377 in read_ndx_and_attrs (f_in=4, f_out=6,
iflag_ptr=0x7ffd814c0f28, type_ptr=0x7ffd814c0f27 "\200\b",
     buf=0x7ffd814c1fc0 "", len_ptr=0x7ffd814c0f2c) at rsync.c:320
 #7  0x00007f87d88c5b5e in recv_files (f_in=5, f_out=-2125728480,
local_name=0x7ffd814c0fc0 "some_dbase") at receiver.c:548
 #8  0x00007f87d88d0787 in do_recv (f_in=4, f_out=6, local_name=0x0) at
main.c:906
 #9  0x00007f87d88d1218 in client_run (f_in=4, f_out=4, pid=-1,
argc=<optimized out>, argv=<optimized out>) at main.c:1213
 #10 0x00007f87d88b4330 in start_client (argv=<optimized out>,
argc=<optimized out>) at main.c:1382
 #11 main (argc=2, argv=0x7f87d9000240) at main.c:1651

Host B (sender):
 #0  0x00007f9a7130a873 in __select_nocancel () at
../sysdeps/unix/syscall-template.S:81
 #1  0x00007f9a71e447a2 in perform_io (needed=needed at entry=4, flags=flags at
entry=17) at io.c:742
 #2  0x00007f9a71e45e2e in raw_read_buf (buf=0x7ffe7c1aa380 "", len=4)
at io.c:895
 #3  0x00007f9a71e47643 in read_buf (f=<optimized out>, buf=buf at
entry=0x7ffe7c1aa380 "", len=len at entry=4) at io.c:1840
 #4  0x00007f9a71e47851 in read_int (f=<optimized out>) at io.c:1711
 #5  0x00007f9a71e2f652 in receive_sums (f=<optimized out>) at sender.c:96
 #6  send_files (f_in=4, f_out=1930300096) at sender.c:332
 #7  0x00007f9a71e37f6c in do_server_sender (argv=<optimized out>,
argc=<optimized out>, f_out=3, f_in=3) at main.c:840
 #8  start_server (f_in=3, f_out=3, argc=2, argv=0x7f9a7302f240) at main.c:1107
 #9  0x00007f9a71e5a34e in rsync_module (f_in=3, f_out=3, i=0,
addr=0x7f9a72098518 <numeric_ids> "\377\377\377\377",
     host=0x7f9a72098c80 <name_buf> "host-a.example.com") at
clientserver.c:1009
 #10 0x00007f9a71e5b260 in start_daemon (f_in=3, f_out=3) at clientserver.c:1100
 #11 0x00007f9a71e4f5d1 in start_accept_loop (port=4, fn=0x7ffe7c1aa160, fn at
entry=0x7f9a71e5b120 <start_daemon>) at socket.c:618
 #12 0x00007f9a71e5bb46 in daemon_main () at clientserver.c:1202
 #13 0x00007f9a71e1aeb1 in main (argc=0, argv=0x0) at main.c:1626
 
The worrysome part is the Send-Q on Host A, which last time contained 263392
unACK'd bytes. The rest of the queues involved were empty. tcpdump 
reveals no retransmission attempts, only 2 window updates and a stream 
of TCP keepalives after the point where the transfer halts:

 Bytes  Time (s)       SRC           DST        Proto    Details
 12995  1.637946       hostb        hosta       RSYNC    Server MOTD
 86     1.637963       hosta       hostb        TCP      39115→rsync(873) [ACK]
Seq=1 Ack=37686543 Win=23993 Len=0 TSval=2255190023 TSecr=100923227
 86     1.638406       hostb        hosta       TCP      [TCP Window Update]
rsync(873)→39115 [ACK] Seq=37686543 Ack=1 Win=1688 Len=0 TSval=100923227
TSecr=2255190022
 1514   1.641767       hostb        hosta       RSYNC    Server MOTD
 86     1.641802       hosta       hostb        TCP      39115→rsync(873) [ACK]
Seq=1 Ack=37687971 Win=23985 Len=0 TSval=2255190024 TSecr=100923228
 1381   1.641938       hostb        hosta       RSYNC    Server MOTD
 86     1.641955       hosta       hostb        TCP      39115→rsync(873) [ACK]
Seq=1 Ack=37689266 Win=23977 Len=0 TSval=2255190024 TSecr=100923228
 86     1.647888       hostb        hosta       TCP      [TCP Window Update]
rsync(873)→39115 [ACK] Seq=37689266 Ack=1 Win=6840 Len=0 TSval=100923230
TSecr=2255190024
 86     121.931963     hosta       hostb        TCP      [TCP Keep-Alive]
39115→rsync(873) [ACK] Seq=0 Ack=37689266 Win=24576 Len=0 TSval=2255220097
TSecr=100923230
 86     121.932062     hostb        hosta       TCP      [TCP Window Update]
rsync(873)→39115 [ACK] Seq=37689266 Ack=1 Win=8467 Len=0 TSval=100953302
TSecr=2255190024
 86     242.248005     hosta       hostb        TCP      [TCP Keep-Alive]
39115→rsync(873) [ACK] Seq=0 Ack=37689266 Win=24576 Len=0 TSval=2255250176
TSecr=100953302
 86     242.248090     hostb        hosta       TCP      [TCP Keep-Alive ACK]
rsync(873)→39115 [ACK] Seq=37689266 Ack=1 Win=8467 Len=0 TSval=100983381
TSecr=2255190024
 86     362.567974     hosta       hostb        TCP      [TCP Keep-Alive]
39115→rsync(873) [ACK] Seq=0 Ack=37689266 Win=24576 Len=0 TSval=2255280256
TSecr=100983381
 86     362.568085     hostb        hosta       TCP      [TCP Keep-Alive ACK]
rsync(873)→39115 [ACK] Seq=37689266 Ack=1 Win=8467 Len=0 TSval=101013461
TSecr=2255190024
 86     482.891958     hosta       hostb        TCP      [TCP Keep-Alive]
39115→rsync(873) [ACK] Seq=0 Ack=37689266 Win=24576 Len=0 TSval=2255310336
TSecr=101013461
 86     482.892026     hostb        hosta       TCP      [TCP Keep-Alive ACK]
rsync(873)→39115 [ACK] Seq=37689266 Ack=1 Win=8467 Len=0 TSval=101043542
TSecr=2255190024
>From the backtraces it appears as though the sender is waiting for thechecksums the client has sent, while the client is waiting for the next file id
to come in. I can only assume the deadlock is because of the client's
checksums
sitting in its Send-Q. The only potentially related part I can find in the
traffic captures are a couple of Zero Window responses by hostb, which however
are followed by window updates with non-zero sizes.

Any ideas or hints what else to look for?

Regards,
Apollon
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