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

Mon Apr 27 21:09:31 UTC 2015

On 04/27/2015 01:52 PM, Ben Turner wrote:
> ----- Original Message -----
>> From: "Ernie Dunbar" <maillist at lightspeed.ca>
>> To: "Gluster Users" <gluster-users at gluster.org>
>> Sent: Monday, April 27, 2015 4:24:56 PM
>> Subject: Re: [Gluster-users] Disastrous performance with rsync to mounted Gluster volume.
>>
>> 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
> This is correct, there is overhead that happenswith small files and  the smaller the file the less throughput you get.  That said, since files are smaller you should get more files / second but less MB / second.  I have found that when you go under 16k changing files size doesn't matter, you will get the same number of 16k files / second as you do 1 k files.

The overhead happens regardless. You just notice it more when you're 
doing it a lot more frequently.

>
>>
>> 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.
> Dragons be here!  Access time is not kept in sync across the replicas(IIRC, someone correct me if I am wrong!) and each time a dir is read from a different brick I bet the access time is different.
>
>> 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.
> It may have something to do with the access times not being in sync across replicated pairs.  Maybe some has experience with this / could this be tripping up rsync?
>
> -b
>
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