On 10.02.20 16:21, Strahil Nikolov wrote:
On February 10, 2020 2:25:17 PM GMT+02:00, Matthias Schniedermeyer <matthias-gluster-users@xxxxxxxxxxxxx> wrote:
Hi
I would describe our basic use case for gluster as:
"data-store for a cold-standby application".
A specific application is installed on 2 hardware machines, the data is
kept in-sync between the 2 machines by a replica-2 gluster volume.
(IOW: "RAID 1")
At any one time only 1 machine has the volume mounted and the
application running. If the machine goes down the application is
started
on the remaining machine.
IOW at any one point in time there is only ever 1 "reader & writer"
running.
I profiled a performance problem we have with this application, which
unfortunately we can't modify.
The profile shows many "opendir/readdirp/releasedir" cycles, the
directory in question has about 1000 files and the application "stalls"
for several milliseconds any time it decides to do a readdir.
The volume is mounted via FUSE and it appears that said operation is
not
cached at all.
To provide a test-case i tried to replicate what the application does.
The problematic operation is nearly perfectly emulated just by using
"ls .".
I created a script that replicates how we use gluster and demonstrates
that a FUSE-mount appears to be lacking any caching of readdir.
A word about the test-environment:
2 identical servers
Dual Socket Xeon CPU E5-2640 v3 (8 cores, 2.60GHz, HT enabled)
RAM: 128GB DDR4 ECC (8x16GB)
Storage: 2TB Intel P3520 PCIe-NVMe-SSD
Network: Gluster: 10GB/s direct connect (no switch), external: 1Gbit/s
OS: CentOS 7.7, Installed with "Minimal" ISO, everything: Default
Up2Date as of: 2020-01-21 (Kernel: 3.10.0-1062.9.1.el7.x86_64)
SELinux: Disabled
SSH-Key for 1 -> 2 exchanged
Gluster 6.7 packages installed via 'centos-release-gluster6'
see attached: gluster-testcase-no-caching-of-dir-operations-for-fuse.sh
The meat of the testcase is this:
a profile of:
ls .
vs:
ls . . . . . . . . . .
(10 dots)
cat /root/profile-1-times | grep DIR | head -n 3
0.00 0.00 us 0.00 us 0.00 us 1
RELEASEDIR
0.27 66.79 us 66.79 us 66.79 us 1 OPENDIR
98.65 12190.30 us 9390.88 us 14989.73 us 2
READDIRP
cat /root/profile-10-times | grep DIR | head -n 3
0.00 0.00 us 0.00 us 0.00 us 10
RELEASEDIR
0.64 108.02 us 85.72 us 131.96 us 10 OPENDIR
99.36 8388.64 us 5174.71 us 14808.77 us 20
READDIRP
This testcase shows perfect scaling.
10 times the request, results in 10 times the gluster-operations.
I would say ideally there should be no difference in the number of
gluster-operations, regardless of how often a directory is read in a
short amount of time (with no changes in between)
Is there something we can do to enable caching or otherwise improve
performance?
Hi Matthias,
Have you tried the 'readdir-ahead' option .
> According to docs it is useful for ' improving sequential directory read performance' .
> I'm not sure how gluster defines sequential directory read, but it's worth trying.
readdir-ahead is enabled by default. Has been for several years.
In effect this option changes how many READDIRP OPs are executed for a
single "ls .".
(It takes more OPs when readdir-ahead is disabled.)
Also, you can try metadata caching , as described in:
https://access.redhat.com/documentation/en-us/red_hat_gluster_storage/3.3/html/administration_guide/sect-directory_operations
The actual group should contain the following:
https://github.com/gluster/glusterfs/blob/master/extras/group-metadata-cache
Metadata-Caching, in general, works:
e.g. `stat FILE` is cached if executed repeatedly.
AFAICT the big exception to metadata-caching is readdir.
--
Matthias
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