[Gluster-users] Disastrous performance with rsync to mounted Gluster volume.

[Ernie Dunbar]

On 2015-04-24 11:43, Joe Julian wrote:

>> This should get you where you need to be.  Before you start to migrate 
>> the data maybe do a couple DDs and send me the output so we can get an 
>> idea of how your cluster performs:
>> 
>> time `dd if=/dev/zero of=<gluster-mount>/myfile bs=1024k count=1000; 
>> sync`
>> echo 3 > /proc/sys/vm/drop_caches
>> dd if=<gluster mount> of=/dev/null bs=1024k count=1000
>> 
>> If you are using gigabit and glusterfs mounts with replica 2 you 
>> should get ~55 MB / sec writes and ~110 MB / sec reads.  With NFS you 
>> will take a bit of a hit since NFS doesnt know where files live like 
>> glusterfs does.

After copying our data and doing a couple of very slow rsyncs, I did 
your speed test and came back with these results:

1048576 bytes (1.0 MB) copied, 0.0307951 s, 34.1 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=1024 bs=1024; sync
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB) copied, 0.0298592 s, 35.1 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=1024 bs=1024; sync
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB) copied, 0.0501495 s, 20.9 MB/s
root at backup:/home/webmailbak# echo 3 > /proc/sys/vm/drop_caches
root at backup:/home/webmailbak# # dd if=/mnt/testfile of=/dev/null 
bs=1024k count=1000
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.0124498 s, 84.2 MB/s


Keep in mind that this is an NFS share over the network.

I've also noticed that if I increase the count of those writes, the 
transfer speed increases as well:

2097152 bytes (2.1 MB) copied, 0.036291 s, 57.8 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=2048 bs=1024; sync
2048+0 records in
2048+0 records out
2097152 bytes (2.1 MB) copied, 0.0362724 s, 57.8 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=2048 bs=1024; sync
2048+0 records in
2048+0 records out
2097152 bytes (2.1 MB) copied, 0.0360319 s, 58.2 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=10240 bs=1024; sync
10240+0 records in
10240+0 records out
10485760 bytes (10 MB) copied, 0.127219 s, 82.4 MB/s
root at backup:/home/webmailbak# dd if=/dev/zero of=/mnt/testfile 
count=10240 bs=1024; sync
10240+0 records in
10240+0 records out
10485760 bytes (10 MB) copied, 0.128671 s, 81.5 MB/s


However, the biggest stumbling block for rsync seems to be changes to 
directories. I'm unsure about what exactly it's doing (probably changing 
last access times?) but these minor writes seem to take a very long time 
when normally they would not. Actual file copies (as in the very files 
that are actually new within those same directories) appear to take 
quite a lot less time than the directory updates.

For example:

# time rsync -av --inplace --whole-file --ignore-existing --delete-after 
gromm/* /mnt/gromm/
building file list ... done
Maildir/                        ## This part takes a long time.
Maildir/.INBOX.Trash/
Maildir/.INBOX.Trash/cur/
Maildir/.INBOX.Trash/cur/1429836077.H817602P21531.pop.lightspeed.ca:2,S
Maildir/.INBOX.Trash/tmp/       ## The previous three lines took nearly 
no time at all.
Maildir/cur/                    ## This takes a long time.
Maildir/cur/1430160436.H952679P13870.pop.lightspeed.ca:2,S
Maildir/new/
Maildir/tmp/                    ## The previous lines again take no time 
at all.
deleting Maildir/cur/1429836077.H817602P21531.pop.lightspeed.ca:2,S
## This delete did take a while.
sent 1327634 bytes  received 75 bytes  59009.29 bytes/sec
total size is 624491648  speedup is 470.35

real	0m26.110s
user	0m0.140s
sys	0m1.596s


So, rsync reports that it wrote 1327634 bytes at 59 kBytes/sec, and the 
whole operation took 26 seconds. To write 2 files that were around 20-30 
kBytes each and delete 1.

The last rsync took around 56 minutes, when normally such an rsync would 
have taken 5-10 minutes, writing over the network via ssh.