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

[Zdenek Kabelac]

Dne 13.9.2017 v 17:33 Dale Stephenson napsal(a):
> 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.

Hi

Can you please provide output of:

lvs -a -o+stripes,stripesize,seg_pe_ranges

so we can see how is your stripe placed on devices ?

SSD typically do needs ideally write 512K chunks.
(something like  'lvcreate -LXXX -i8 -I512k vgname' )

Wouldn't be 'faster' to just concatenate 8 disks together instead of striping 
- or stripe only across 2 disk - and then you concatenate 4 such striped areas...

64k stripes do not seem to look like ideal match in this case of 3 disk with 
512K blocks

Regards

Zdenek