Hi Strahil,
cluster.lookup-optimize had been default turn on since i think is gluster version 6 corresponding in oVirt 4.3, so ovirt inherit this setting regardless of ovirt preset. My volumes are provisioned by ovirt UI.
Yes, In the theory, shards improve read performance, however write doesn't, it do slowing down in general.
In my lab test environment, 4 x 1.8TB SAS RAID 10 (2 +1 arbiter) JBOD, 10GbE
I do get 400MB Read but 70MB sequential write in shard configuration with 1M4Q 70R/30W, but I do get 380MB read 120MB write without shards. In 8k16Q 70R/30W workload, i got double performance when not using shard. Thats will let me re-think about theoretical vs real-world scenario.
For the quote of sharding use case from RedHat, yes I've verified that when NO sharding, it will lose live storage migration flexibility. Thank for pointing out this.
https://lists.ovirt.org/archives/list/users@ovirt.org/message/ZZDWJAVBLNDGFRU2ENN5UZ35Q4PZUS5C/