Tag: storage

2.5 VS 3.5 HDD: What Are the Differences and Which One Is Better? – MiniTool

A hard drive is a piece of hardware used to store digital content and data on computers. All computers requires secondary storage to store data on a long term and not lost in case of a power loss. Secondary storage devices are of two types: hard disk drives (HDD) and solid-state drives (SSD).

Hard disk drives are made up of magnetized disks—known as platters—that spin rapidly, typically somewhere between 5,400 and 15,000 RPM.
The faster the magnetic disk rotates, the quicker your computer is able to access information from it.

Hard drives generally come into two different form factors which are 2.5-inch and 3.5-inch. The form factor of hard disks denotes the diameter of the disk platter inside the hard disk.

2.5 inch vs 3.5 inch
1. The size will be 3 inch and 4 inch respectively.
2. 2.5 is used in laptops, desktops and servers while 3.5 inch in desktop and servers.
3. 2.5 inch draws less power compared to 3.5 inch due to the moving mechanical parts.
4. 3.5 inch will have a higher cache size.
5. 3.5 inch will have higher RPM
6. 3.5 inch will have a higher capacity.
7. 3.5 inch can transfer more data as the number of sectors per track in a disk platter is more. For same spin speed 3.5 inch will perform better.
8. Due to tighter and smaller construction as well as complex design 2.5 inch will be expensive.
9. 2.5 inch is added with accerelometers which can detect shock and park its head. This minimizes the impact.
10. 2.5 inch can be used in a 3.5 inch adaptor.

Source: 2.5 VS 3.5 HDD: What Are the Differences and Which One Is Better? – MiniTool

Understanding write-through, write-around and write-back caching (with Python)

Write-through is used when the data is written to both cache and persistent storage. Useful for less intensive writes and read intensive applications.

Write-back is used when data is written to cache and a background offload will processor or application will be used to copy the data from the cache to persistent storage.  IO will be acknowledged on the cache write itself. low read latency and high throughput for write intensive applications. Cache may need to be powered with an alternate source or cache to be duplicated to avoid data loss on power loss or cache failure.

Write-around is used when cache is bypassed and directly written to the persistent data. Less use of cache and most time cache miss. So higher latency.


Source: Understanding write-through, write-around and write-back caching (with Python)

How to check the disk performance using dd

dd command should be available in ESXi as well as all Linux versions. If you face any issues with your application and wants to know the disk performance is deteriorated, you can quickly use the dd command to check the write/read performance of the disk. The disk used can be even a datastore or a network device also. You can cross check with another system or disk where you don’t see any performance issues to compare the results to know if there is any issues with the disk.

Below is the example to run in a VMWare ESXi volume.

/vmfs/volumes/54a55c07-9face633-0999-002590fc98c8 # time dd if=/dev/zero of=test bs=1M count=1024

1024+0 records in 1024+0 records out

real 2m 7.62s user 0m 4.69s sys 0m 0.00s

= ( 1024/127 = 8 MB/s !!! )

In the above example you are trying to write a 1GB file and you are using time command to find out the speed at which the write happened.

Source: Horrible disk performance under ESXi | [H]ard|Forum