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

[Joe Julian]

On 04/27/2015 03:00 PM, Ernie Dunbar wrote:
> On 2015-04-27 14:09, Joe Julian wrote:
>>
>> 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 happens with 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.
>
>
> Well, it would be helpful to know what specifically rsync is trying to 
> do when it's sitting there making overhead, and whether it's possible 
> to tell rsync to avoid doing it, and just copy files instead (which it 
> does quite quickly).
>
> I suppose technically speaking, it's an rsync-specific question, but 
> it's all about making rsync and glusterfs play nice, and we pretty 
> much all need to know that!

Yes, that's very rsync specific. Rsync not only checks the files 
metadata, but it also does a hash comparison.

Each lookup() of each file requires a lookup from *each* replica server. 
Lookup's are triggered on open, or even fstat. Since rsync requests the 
stat of every file for comparison, this requires a little extra network 
time. The client queries all the replica in case one is out of date to 
ensure it's returning accurate results (and heal if a replica needs it).

After bulding a list of files that differ between the source and the 
target, rsync copies the file to a temporary filename. After completing 
the temporary file, rsync then renames the temporary to the target 
filename. This has the disadvantage of putting the target file on the 
"wrong" dht subvolume because the hash for the temporary filename is 
different from the target filename.

rsync's network transfer is over ssh by default, so you're also hitting 
encryption and buffer overhead.

The optimum process for *initially* copying large numbers of files to 
gluster would be to blindly copy a list of files from the source to the 
target without reading the target. If copying across a network, 
maximizing the packet size is also advantageous. I've found tar ( + pv 
if you want to monitor throughput) + netcat (or socat) to be much faster 
than rsync.