[linux-lvm] Performance penalty for 4k requests on thin provisioned volume

Dale Stephenson dalestephenson at mac.com
Wed Sep 13 15:33:05 UTC 2017

Distribution: centos-release-7-3.1611.el7.centos.x86_64
Kernel: Linux 3.10.0-514.26.2.el7.x86_64
LVM: 2.02.166(2)-RHEL7 (2016-11-16)

Volume group consisted of an 8-drive SSD (500G drives) array, plus an additional SSD of the same size.  The array had 64 k stripes.
Thin pool had -Zn option and 512k chunksize (full stripe), size 3T with metadata volume 16G.  data was entirely on the 8-drive raid, metadata was entirely on the 9th drive.
Virtual volume “thin” was 300 GB.  I also filled it with dd so that it would be fully provisioned before the test.
Volume “thick” was also 300GB, just an ordinary volume also entirely on the 8-drive array.

Four tests were run directlyagainst each volume using fio-2.2.8, random read, random write, sequential read, sequential write.  Single thread, 4k blocksize, 90s run time.

The thin volume should much worse performance in this test:

Random read : io=51607MB, bw=587168KB/s, iops=146792, runt= 90001msec
Random write: io=46530MB, bw=529406KB/s, iops=132351, runt= 90001msec
Sequential read : io=176561MB, bw=1961.7MB/s, iops=10462, runt= 90006msec
Sequential write: io=162451MB, bw=1804.1MB/s, iops=9626, runt= 90006msec

Random read : io=88350MB, bw=981.68MB/s, iops=251303, runt= 90001msec
Random write: io=77905MB, bw=886372KB/s, iops=221592, runt= 90001msec
Sequential read : io=89095MB, bw=989.96MB/s, iops=253421, runt= 90001msec
Sequential write: io=77742MB, bw=884520KB/s, iops=221130, runt= 90001msec

As you can see, the number of iops for the thin-provisioned volume is dramatically less than for thick volumes.  With the default 64k chunk size this is also true.

Running the same fio test with 512k request size shows similar performance for thick and thin volumes, so the maximum potential throughput of the thin volume seems broadly similar to thin volumes.  But I’d like the maximum number of iops to at least be within shouting distance of thick volumes.  Is there anything that can be done to improve that with either this version of lvm/device-mapper or later versions?  I typically use xfs on top of volumes, and xfs is very fond of 4k requests.

Dale Stephenson

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