[Bugs] [Bug 1557904] New: [EC] Read performance of EC volume exported over gNFS is significantly lower than write performance

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1557904

            Bug ID: 1557904
           Summary: [EC] Read performance of EC volume exported over gNFS
                    is significantly lower than write performance
           Product: GlusterFS
           Version: 3.13
         Component: disperse
          Assignee: bugs at gluster.org
          Reporter: aspandey at redhat.com
                CC: bugs at gluster.org
        Depends On: 1554743



+++ This bug was initially created as a clone of Bug #1554743 +++

Description of problem:
Reads are only at 47MB/s while writes are at 219MB/s:



dd if=/dev/zero of=/media1/results/results/test-toberemoved/test.bin bs=1M
count=1000 conv=fdatasync
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 4.785 s, 219 MB/s

echo 3 > /proc/sys/vm/drop_caches

dd if=/media1/results/results/test-toberemoved/test.bin of=/dev/null bs=1M
count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB) copied, 22.1433 s, 47.4 MB/s
================================================================================

Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
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Actual results:


Expected results:


Additional info:

--- Additional comment from Worker Ant on 2018-03-13 05:47:14 EDT ---

REVIEW: https://review.gluster.org/19703 (cluster/ec: Change default read
policy to gfid-hash) posted (#2) for review on master by Ashish Pandey

--- Additional comment from Worker Ant on 2018-03-14 06:10:44 EDT ---

COMMIT: https://review.gluster.org/19703 committed in master by "Ashish Pandey"
<aspandey at redhat.com> with a commit message- cluster/ec: Change default read
policy to gfid-hash

Problem:
Whenever we read data from file over NFS, NFS reads
more data then requested and caches it. Based on the
stat information it makes sure that the cached/pre-read
data is valid or not.

Consider 4 + 2 EC volume and all the bricks are on
differnt nodes.

In EC, with round-robin read policy, reads are sent on
different set of data bricks. This way, it balances the
read fops to go on all the bricks and avoid heating UP
(overloading) same set of bricks.

Due to small difference in clock speed, it is possible
that we get minor difference for atime, mtime or ctime
for different bricks. That might cause a different stat
returned to NFS based on which NFS will discard
cached/pre-read data which is actually not changed and
could be used.

Solution:
Change read policy for EC as gfid-hash. That will force
all the read to go to same set of bricks.

Change-Id: I825441cc519e94bf3dc3aa0bd4cb7c6ae6392c84
BUG: 1554743
Signed-off-by: Ashish Pandey <aspandey at redhat.com>


Referenced Bugs:

https://bugzilla.redhat.com/show_bug.cgi?id=1554743
[Bug 1554743] [EC] Read performance of EC volume exported over gNFS is
significantly lower than write performance
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