When you want to resolve problems with storage quickly, you must understand the architecture. There is a big difference between NFS and VMFS filesystems, or if DAS, FC, FCoE, iSCSI, or the new vSAN or VVOL are used. Two great friends will be esxcli storage and esxcli iscsi for particular use with iSCSI storage.

A problem you may need to analyze is the space occupied on the datastore. Use the df -h command to do so:

[root@esxi-prod-1:/var/log] cd /vmfs/volumes/
[root@esxi-prod-1:/vmfs/volumes] df -h
Filesystem Size   Used    Available Use%   Mounted on
VMFS-6    499.8G  135.0G  364.8G    27%    /vmfs/volumes/Shared Storage
VMFS-6    49.8G   5.5G    44.2G     11%    /vmfs/volumes/C1
VMFS-6    49.8G   9.4G    40.4G     19%    /vmfs/volumes/C2
VMFS-6    49.8G   5.6G    44.1G     11%    /vmfs/volumes/C3
VMFS-6    49.8G   1.6G    48.1G     3%     /vmfs/volumes/C4
vfat      249.7M  155.3M  94.4M     62%    /vmfs/volumes/c931af73-
                                          7283cfde-5f11-b0b5ca4e48fc
vfat     285.8M   174.2M  111.6M    61%    /vmfs/volumes/5c60274d-
                                         50968070-edd0-005056a95cde
vfat     4.0G     19.8M   4.0G      0%    /vmfs/volumes/5c602753-
                                         c9175297-8143-005056a95cde
vfat     249.7M   155.3M 94.4M     62%    /vmfs/volumes/4a41ca7c-
                                         4c35da2c-e33e-88f6e0f74792