By Greg Schulz, Server and StorageIO @storageio
In part II of this series we covered some of the differences between various Hard Disk Drive (HDD) including looking beyond the covers at availability, cache and cost. Let us pick up where we left off on our look beyond the covers to help answer the question of which is the best HDD to use.
Form factor (physical attributes)
Physical dimensions including 2.5” small form factor (SFF) and 3.5” large form factor (LFF) HDDs. 2.5” HDDs are available in 7mm, 9mm and larger 14mm height form factors. Note that taller drives tend to have more platters for capacity. In the following image note that the bottom HDDs is taller than the others are.
The above tall or “thick” (not to be confused with thick or thin provisioned) is a SFF 5.4K RPM 1.5TB drive that I use as an on-site backup or data protection target and buffer. The speed is good enough for what I use it for, and provides good capacity per cost in a given footprint.
Also, note that there is a low profile 7mm device (e.g. middle) that for example can fit into my Lenovo X1 laptop as a backup replacement for the Samsung SSD that normally resides there. Also shown on the top is a standard 9mm height 7.2K Momentus XT HHDD with 4GB of slc nand flash and 500GB of regular storage capacity.
Functionality include rebuild assist, secure erase, self-encrypting device (SEDs) without or without FIPS, RAID assist, support for large file copy (e.g. for cloud, object storage and dispersal or erasure code protection). Other features include intelligent power management (beyond first generation MAID), native command queue (NCQ), and Advanced Format (AF) 4Kbyte block and 512 byte emulation). Features also include those for high-density deployments such as virtualization and cloud such as vibration management in addition to SMART (Self-Monitoring, Analysis, and Reporting Technology) reporting and analysis.
Drives can also depending on vendor, make and model support various block or sector sizes including standard 512, 520, 524 and 528 for different operating systems, hypervisors or controllers. Another feature mentioned above is the amount of volatile (DRAM) or persistent (nand flash) cache for read and read-ahead. Some drives are optimized for standalone or JBOD (Just a Bunch of Disks) and others for use with RAID controllers. By the way, put several SSD drives into an enclosure without a controller and you have Just a Bunch Of SSDs or JBOS. What this means is that some drives are optimized to work with RAID arrays and how they chunk or shard data while others are for non-RAID use.
Speaking of RAID and HDDs, have you thought about your configuration settings, particular if working with big data or big bandwidth and large files or objects? If not you should including paying attention to stripe, chunk or shard size of how much data gets written to each device. With larger IO sizes, revisit what the default settings are to determine if you need to make some adjustments. Just as some drives are optimized for working with RAID controllers or software, there are some drives being optimized for cloud and object storage along with big data applications. The differences is that these drives are optimized for moving larger chunks or amounts of data usually associated with distributed data dispersal, erasure coding and enhanced RAID solutions. An example of a cloud storage optimized HDD is the Seagate Constellation CS (Cloud Storage).
Moving on, some drives are designed to be spinning or in constant use while others for starting and stopping such as with a notebook or desktop. Other features appearing in HDDs support high-density, along with hot and humid environments for cloud and managed service provider or big data needs. The various features and functionality can be part of the firmware enabled for a particular device along with hard features built into the device.
Interface type and speed
The industry trend is moving towards 6Gb SAS for HDDs similar to that for SSD drives. However, there is also plenty of 6Gb SATA activity, along with continued 4Gb Fibre Channel (4GFC) that eventually will transition to SAS. There is also prior generation 3Gb SAS and 3Gb SATA and you might even have some older 1.5Gb SAS or SATA devices around, maybe even some Parallel ATA (PATA) or Ultra320 (Parallel SCSI). Note that SATA devices can plug into and work with SAS adapters and controllers, however not the other way around.
Note that if you see or hear about a storage system or controller with back-end 8Gb Fibre Channel, chances are the HDD would auto-throttle negotiate down to 4GFC. In addition to the current 6Gb speed of SAS, there are improvements in the works for 12Gb and beyond, along with many different topology or configuration options. If you are interested in learning more about SAS, check out SAS SANs for Dummies sponsored by LSI that I helped write.
Notice I did not mention iSCSI, USB, Thunderbolt or other interfaces and protocols? Some integrators and vendors offer drives with those among other interfaces, they are usually SAS or SATA with a bridge, router or converter interface attached to them externally or as part of their packaging (See following image).
Performance of the device
A common high-level gauge of drive performance is the platter rotational speed. However there is other metrics including seek time, transfer rate and latency. These in turn vary based on peak and sustained, read or write, random or sequential, large or small IOPS or transfer requests. There are many different numbers floating around as to how many IOPS a HDD can do based on its rotational speed among other factors. The challenge with these numbers or using them is putting into context of what size the IOP is, was it a read or write, large or small, random or sequential relative to your needs. Another challenge is how those IOPs are measured, for example were the measured below a file system to negate buffering, or via a file system.
Rotational speed such as 5,400 (5.4K) revolutions per minute (RPM), 7.2K, 10K and 15K RPMs. Note that while a general indicator of relative speed, some of the newer 10K SFF (e.g. 2.5”) HDDs provide the same or better performance of earlier generation 3.5” 15K devices. This is accomplished with a combination of smaller form factor (spiral transfer rate) and improvements in read/write electronics and firmware. The benefit is that in the same or smaller footprint, more devices, performance and capacity can be packaged as well as the devices individually using less power. Granted if you pack more devices into a given footprint, the aggregate power might increase, however so too does the potential performance, availability, capacity and economics depending on implementation. You can see the differences in performance using various HDDs including an HHDD in this post here that looked at Windows impact for VDI planning.
This wraps up this post, up next part IV, we continue our look beyond the covers to determine the differences and what HDD is best for your virtual or other data storage needs.
Ok, nuff said (for now).