Last Updated: 2023-07-11

Memory vs SSD

  • Memory: volatile, byte-addressable; Modern DDR4 DIMMs have latencies under 15ns.
  • SSD: persistent, block-addressable.

Form Factors

  • Small Form-Factor (SFF): 2.5-inch.
  • Large Form-Factor (LFF): 3.5-inch.


Most SSD manufacturers use non-volatile NAND flash memory in the construction of their SSDs because of the lower cost compared with DRAM and the ability to retain the data without a constant power supply.

Intel® Optane™ SSD is based on a new non-volatile memory technology called 3D XPoint. 3D XPoint is more expensive than NAND, but cheaper than DRAM.

Intel discontinues Optane in 2022, industry shift to CXL (Compute Express Link)-based architectures.

SSD,其实包含了两部分,一部分是 NAND Flash 颗粒,另一部分则是控制器。 每个 SSD 都是一个独立的「设备」,并拥有 SATA 或者 M.2 插口以方便在现代的 PC 上互换。

SLC vs MLC vs TLC vs QLC

  • data stored in semiconductor cells, one cell can contain between 1 and 4 bits of data
    • SLC: single bit cells, the most reliable, durable, fast, and expensive
    • MLC: 2 bits
    • TLC: 3 bits
    • QLC: 4 bits
    • PLC: 5 bits


Used in cloud, e.g. for data backup.

Magnetic Storage Technology

  • Perpendicular magnetic recording (PMR): conventional hard disk drives record data by writing non-overlapping magnetic tracks parallel to each other
  • Shingled magnetic recording (SMR): increase storage density and overall per-drive storage capacity. shingled recording writes new tracks that overlap part of the previously written magnetic track, leaving the previous track narrower and allowing for higher track density. Thus, the tracks partially overlap similar to roof shingles. This approach was selected because, due to physical limitations, recording magnetic heads are wider than reading heads. All 2.5 inch HDDs over 1 TB are SMR.
  • Heat-assisted magnetic recording (HAMR): Seagate; temporarily heating the disk material during writing, which makes it much more receptive to magnetic effects and allows writing to much smaller regions (and much higher levels of data on a disk). 20 TB HAMR drives were released in January 2021.
  • Energy-Assisted Magnetic Recording (EAMR): Western Digital.

Power Supply

  • 3.5 inch HDD uses 12 volts DC, and 5 volts DC.
  • 2.5 inch HDD will only use the 5 VDC supply



SATA: Serial Advanced Technology Attachment. SATA replaces PATA (Parallel ATA).

SATA Cables: one end plugs into a port on the motherboard, the other end into the back of a storage device.


NVM Express (NVMe) : use PCIe bus instead of SATA for SSD, reduces I/O overhead and brings various performance improvements relative to previous logical-device interfaces, including multiple, long command queues, and reduced latency.

The older SATA drives allow the transfer of information only along one channel, NVMe makes use of multiple channels that can read and write at the same time.


NVMe: storage drives (and other components) can be connected directly to the PCIe interface, which allows the SSD to communicate directly with the CPU instead of going through the motherboard as SATA previously required.

NVMe is much faster than SATA:

  • NVMe PCIe 3.0: up to 3.5 GB / second
  • NVMe PCIe 4.0: up to 7.5 GB / second
  • SATA SSD: up to 500 MB / second
  • SATA HDD (7,200 RPM): ~160 MB / second

NVMe vs M.2

Not comparable.

  • NVMe is the spec for storage drives.
  • M.2 is a form factor, think of 2.5-inch drives, 3.5-inch drives, and M.2 drives.

M.2 can come with either NVMe or SATA. NVMe SSDs are usually found in the M.2 form factor. SATA SSDs can either take the form of 2.5-inch or M.2 drives.

SD Card


  • SDSC (Standard-Capacity): Up to 2 GB
  • SDHC (High-Capacity): over 2 GB to 32 GB
  • SDXC (eXtended-Capacity): over 32 GB to 2 TB
  • SDUC (Ultra-Capacity): over 2 TB to 128 TB

SDXC and SDUC cards are normally formatted using the exFAT file system.

SDXC is now the mainstream. Macbook Pro has a built-in SDXC slot.

Bus speed:

Default Speed < High Speed < UHS I < UHS II < UHS III < SD Express (>985 MB/s)

(UHS = Ultra High Speed)

App Performance Class

There are two ratings for the App Performance Class which are known as A1 and A2.

  • A1: a minimum random read of 1500 IOPS and a minimum random write of 500 IOPS
  • A2: a minimum random read of 4000 IOPS and a minimum random write of 2000 IOPS.

Speed Class

Min Write Speed Speed Class UHS Speed Class Video Speed Class
10 MB/s Class 10 (C10) Class 1 (U1) Class 10 (V10)
30 MB/s Class 10 (C10) Class 3 (U3) Class 30 (V30)
60 MB/s Class 10 (C10) Class 3 (U3) Class 60 (V60)
90 MB/s Class 10 (C10) Class 3 (U3) Class 90 (V90)

Portable External Hard Drive

How to format for both Windows and Mac

By default Windows uses NTFS while Mac OS uses APFS (new) and HFS+ (old), and they're incompatible with each other. In order to be used by both, use exFAT filesystem.


2.5 inch, up to 5 TB:

  • Seagate
  • WD
    • WD Elements Portable: inexpensive, no extra features
    • WD My Passport: with auto backup, hardware encryption, passpord protection
    • WD Black: high performance, more expensive


S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) attributes: to detect and report various indicators of drive reliability with the intent of anticipating imminent hardware failures.

  • Power-on hours: indicates a remaining lifetime prediction for HDD / SSD ("the total expected life-time of a hard disk is 5 years" or 43,800 hours of constant use.)


  • wmic diskdrive get status: Windows built-in, however only reporting status.
  • SeaGate: SeaTools



ONTAP: NetApp’s proprietary operating system used in storage disk arrays such as NetApp FAS/AFF...

  • FAS: Fabric-Attached Storage
    • NetApp proprietary custom-build hardware appliances with HDD or SSD drives called hybrid FAS.
    • AFF: All Flash FAS. File / block storage. NetApp proprietary custom-build hardware appliances with only SSD drives and optimized ONTAP for low latency called AFF.
      • https://www.netapp.com/data-storage/aff-a-series/
      • 2U: AFF A250, entry-level
      • 4U: AFF A400, mid-range
      • 4U: AFF A800, for performance-hungry apps, low latency (smaller max capacity comapring to A400 though)
      • 8U: AFF A900, new powerhouse of the A-Series, for mission-critical apps

The terms ONTAP, AFF, ASA, FAS are often used as synonyms.

It enables you to combine multiple physical storage controllers into a single logical cluster that can non-disruptively service multiple storage workload needs.

All SAN Array build on top of AFF platform, and provide only SAN-based data protocol connectivity.

NetApp StorageGrid

2 primary usages:

  1. stand-alone to provide S3 object storage, S3 compatible APIs.
  2. as cold storage for AFF.


  • SG1000 / SG100: gateway node, admin node, load balancer, HA.
  • SG6000 Series
    • SG6000-CN: controller, deployed with SG6060.
    • SG6060: for transactional small object storage workloads
  • SG5000 Series: for secondary storage workloads.

AFF vs StorageGrid

  • AFF is all flash (NVMe), StorageGrid may use SSD / HDD
  • Tiered: hot data in AFF; cold data in StorageGrid
    • FabridPool: NetApp ONTAP => StorageGrid