Na co si dat pozor pri navrhu storage

Things that affect scalability

Throughput

* Fibre Channel link speed
* Number of outstanding I/O requests
* Number of disk spindles
* RAID type
* SCSI reservations
* Caching or prefetching algorithms

Latency

* Queue depth or capacity at various levels
* I/O request size
* Disk properties such as rotational, seek, and access delays
* SCSI reservations
* Caching or prefetching algorithms.

Factors affecting scalability of ESX storage

Number of active commands

* SCSI device drivers have configurable parameter called LUN queue depth which determines how many commands can be active to a given LUN at any one time.
* QLogic fibre channel HBAs support up to outstanding commands 256, Emulex 128
* Default value in ESX is set to 32 for both
* Any excess commands are queued in vmkernel which increases latency
* When VMs share a LUN, the total number of outstanding commands permitted from all VMs to that LUN is goverened by Disk.SchedNumReqOutstanding. If this is exceeded, commands will be queued in VMkernel. Maximum figure recommended is 64. For LUNs with single VM, this figure is inapplicable, and HBA queue depth is used.
* Disk.SchedNumReqOutstanding should be the same value as the LUN queue depth.
* n = Maximum Outstanding I/O Recommended for array per LUN (this figure should be obtained with help from the storage vendor)
* a = Average active SCSI Commands per VM to shared VMFS
* d = LUN queue depth on each ESX host
* Max number VMs per ESX host on shared VMFS = d/a
* Max number VMs on shared VMFS = n/a
* To establish a look at QSTATS in esxtop, and add active commands to queued commands to get total number of outstanding commands.

SCSI Reservations

* Reservations are created by creating/deleting virtual disks, extending VMFS volume, creating/deleting snapshots. all these result in metadata updates to the file system using locks.
* Recommendation is to minimise these activities during the working day.
* Perform these tasks on the same ESX host that hosts I/O intensive VMs as the SCSI reservations are issued by the same host as there will be no reservation conflicts as the host is already generating the reservations. I/O intensive VMs on other hosts will be affected for the duration of the task.
* Limit the use of snapshots. It is not recommended to run many virtual machines from multiple servers that are using virtual disk snapshots on the same VMFS. Snapshot files grow in 16MB chunks, so for vmdks with lots of changes, this file will grow quickly, and for every 16MB chunk that the file grows by, you will get a SCSI reservation.

Zdroj:
Andy Troup, employed by VMware as a Senior Consultant and am the EMEA Strategy & Operations Practice Lead. http://virtuallyandy.blogspot.com/2009/03/storage-best-practice.html
Scalable Storage Performance http://www.vmware.com/files/pdf/scalable_storage_performance.pdf