20 Nov
2020
20 Nov
'20
2:25 p.m.
Hmm, so maybe your hardware is good enough that cache is actually not helping? This is not
unheard of. I don't really see any improvement from caching to begin with. On the
other hand, a synthetic benchmark is not really a test that utilises the good sides of
cache (in particular, write merges will probably not occur). It would probably make more
sense to run real VMs with real workload for a while and monitor latencies etc. over a
longer period of time.
Other than that I only see that the cache size is quite small. You do 100G random
operations on a 16M cache, the default is 32M. I would not expect anything interesting
from this ratio. Cache only makes sense if you have a lot of cache hits.
In addition, the max_dirty and target_dirty values are really high percentage-wise. This
could lead to a lot of deferred operations for too long and result in a cache flush
blocking IO.
Larger cache size, smaller targets for dirty and a benchmark that simulates a realistic
workload might be worth to investigate.
Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
________________________________________
From: norman <norman.kern(a)gmx.com>
Sent: 20 November 2020 13:58:27
To: Frank Schilder
Cc: ceph-users
Subject: Re: [ceph-users] The serious side-effect of rbd cache setting
If the rbd cache = false, and run the same two tests, the read iops is
stable(this is a new cluster without stress):
109 274471 2319.41 9500308.72
110 276846 2380.81 9751782.65
111 278969 2431.40 9959023.39
112 280924 2287.21 9368428.23
113 282886 2227.82 9125145.62
114 286130 2331.61 9550275.83
115 289693 2569.19 10523406.25
116 293161 2838.17 11625140.61
117 296484 3111.75 12745715.04
118 300068 3436.12 14074349.33
119 302424 3258.53 13346958.90
120 304442 2949.56 12081397.86
121 306988 2765.18 11326156.91
122 309867 2676.38 10962461.69
123 312475 2481.20 10162987.53
124 314957 2506.40 10266198.33
125 317124 2536.19 10388249.19
126 320239 2649.98 10854336.06
127 323243 2674.98 10956727.73
128 326688 2842.37 11642342.34
129 328855 2779.37 11384315.33
130 331414 2857.77 11705415.59
131 333811 2714.18 11117277.84
132 336164 2583.99 10584022.02
133 338664 2395.01 9809941.00
134 341417 2512.20 10289953.14
135 344409 2598.79 10644637.88
136 347112 2659.98 10895292.68
137 349486 2664.18 10912494.47
138 351921 2651.18 10859250.80
139 354592 2634.79 10792081.86
140 357559 2629.79 10771603.52
On 20/11/2020 下午8:50, Frank Schilder wrote:
Do you have test results for the same test without
caching?
I have seen periodic stalls in any RBD IOP/s benchmark on ceph. The benchmarks create IO
requests much faster than OSDs can handle them. At some point all queues run full and you
start seeing slow ops on OSDs.
I would also prefer if IO activity was more steady and not so bursty, but for some reason
IO client throttling is pushed to the clients instead of the internal OPS queueing system
(ceph is collaborative, meaning a rogue un-collaborative client can screw it up for
everyone).
If you know what your IO stack can handle without stalls, you can use libvirt QOS
settings to limit clients with reasonable peak-load and steady-load settings.
Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
________________________________________
From: norman <norman.kern(a)gmx.com>
Sent: 20 November 2020 13:40:18
To: ceph-users
Subject: [ceph-users] The serious side-effect of rbd cache setting
Hi All,
We're testing the rbd cache setting for openstack(Ceph 14.2.5 Bluestore
3-replica), and an odd problem found:
1. Setting librbd cache
[client]
rbd cache = true
rbd cache size = 16777216
rbd cache max dirty = 12582912
rbd cache target dirty = 8388608
rbd cache max dirty age = 1
rbd cache writethrough until flush = true
2. Running rbd bench
rbd -c /etc/ceph/ceph.conf \
-k /etc/ceph/keyring2 \
-n client.rbd-openstack-002 bench \
--io-size 4K \
--io-threads 1 \
--io-pattern seq \
--io-type read \
--io-total 100G \
openstack-volumes/image-you-can-drop-me
3. Start another test
rbd -c /etc/ceph/ceph.conf \
-k /etc/ceph/keyring2 \
-n client.rbd-openstack-002 bench \
--io-size 4K \
--io-threads 1 \
--io-pattern rand \
--io-type write \
--io-total 100G \
openstack-volumes/image-you-can-drop-me2
Running for minutes, I found the read test almost hung for a while:
69 152069 2375.21 9728858.72
70 153627 2104.63 8620569.93
71 155748 1956.04 8011953.10
72 157665 1945.84 7970177.24
73 159661 1947.64 7977549.44
74 161522 1890.45 7743277.01
75 163583 1991.04 8155301.58
76 165791 2008.44 8226566.26
77 168433 2153.43 8820438.66
78 170269 2121.43 8689377.16
79 172511 2197.62 9001467.33
80 174845 2252.22 9225091.00
81 177089 2259.42 9254579.83
82 179675 2248.22 9208708.30
83 182053 2356.61 9652679.11
84 185087 2515.00 10301433.50
99 185345 550.16 2253434.96
101 185346 407.76 1670187.73
102 185348 282.44 1156878.38
103 185350 162.34 664931.53
104 185353 12.86 52681.27
105 185357 1.93 7916.89
106 185361 2.74 11235.38
107 185367 3.27 13379.95
108 185375 5.08 20794.43
109 185384 6.93 28365.91
110 185403 9.19 37650.06
111 185438 17.47 71544.17
128 185467 4.94 20243.53
129 185468 4.45 18210.82
131 185469 3.89 15928.44
132 185493 4.09 16764.16
133 185529 4.16 17037.21
134 185578 18.64 76329.67
135 185631 27.78 113768.65
Why this happened? It's a unacceptable performance for read.
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