[Bugs] [Bug 1665029] New: read-ahead and io-cache degrading performance on sequential read

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1665029

            Bug ID: 1665029
           Summary: read-ahead and io-cache degrading performance on
                    sequential read
           Product: GlusterFS
           Version: 5
          Hardware: x86_64
                OS: Linux
            Status: NEW
         Component: read-ahead
          Severity: high
          Assignee: bugs at gluster.org
          Reporter: mpillai at redhat.com
                CC: bugs at gluster.org
  Target Milestone: ---
    Classification: Community



Description of problem:
A large-file sequential read test reports better throughput with read-ahead and
io-cache xlators turned off.

One test is obviously not enough to decide the fate of these xlators. But
through this bz we can answer some of the relevant questions.

Version-Release number of selected component (if applicable):
glusterfs-*5.2-1.el7.x86_64
kernel-3.10.0-957.el7.x86_64 (RHEL 7.6)

How reproducible:
Consistently

Steps to Reproduce:
1. fio write test to generate data set:
fio --name=initialwrite --ioengine=sync --rw=write \
--direct=0 --create_on_open=1 --end_fsync=1 --bs=128k \
--directory=/mnt/glustervol/ --filename_format=f.\$jobnum.\$filenum \
--filesize=16g --size=16g --numjobs=4

2. unmount and re-mount volume on client

3. fio sequential read test to read back the data written in step 1:
fio --name=readtest --ioengine=sync --rw=read \
--direct=0 --bs=128k --directory=/mnt/glustervol/ \
--filename_format=f.\$jobnum.\$filenum --filesize=16g \
--size=16g --numjobs=4

Actual results:

With default volume settings:
READ: bw=581MiB/s (610MB/s), 145MiB/s-146MiB/s (152MB/s-153MB/s), io=64.0GiB
(68.7GB), run=112401-112717msec

With read-ahead and io-cache turned off:
READ: bw=1083MiB/s (1136MB/s), 271MiB/s-271MiB/s (284MB/s-284MB/s), io=64.0GiB
(68.7GB), run=60487-60491msec

So in this test, there is a significant performance gain with these xlators
turned off.

The low performance with default volume settings was first seen in another
bz:https://bugzilla.redhat.com/show_bug.cgi?id=1664934#c0.

Expected results:
performance xlators should not degrade performance

Additional info:

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