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we have recently changed the file server that is the repository (the place where the backups are stored) server .

I noticed the hardware specially IO is much better backup take shorter to finish, depending on many things, however, restores still take the same time as before.

this is not a complex question, I have only one question here:

what is physical_block_size?

when you run the following query on the msdb database:

SELECT * 
  FROM msdb.dbo.backupmediafamily BMF WITH(NOLOCK)
 order by 1 desc

this is the result of above query and please notice how the physical_block_size changed from 4096 to 512 exactly when I decided to use our new server mentioned above for the destination of the backups:

enter image description here

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The backupmediafamily says it's the "Physical block size used to write the media family".

I understand (from research) it relates to the physical specs of the underlying disks of storage you're using. If that's the case, you could verify that info with your storage admin. This Advanced Format article says:

Note: Software and documentation may sometimes refer to "sectors" and "blocks" interchangeably, regardless of the storage type.

Physical sector size
This is the smallest unit a physical storage device claims it can write atomically. For HDDs, it is the actual size of sectors in a platter. Traditionally, the physical sector size for HDDs was 512 bytes, meaning that each sector could hold 512 bytes of data. However, with the introduction of Advanced Format HDDs, the physical sector size was increased to 4096 bytes (4 KiB) for increased storage density and improved error correction capabilities. SSDs do not expose their actual NAND flash memory page size, which typically ranges from 4 KiB to 16 KiB, instead their reported physical sector size is the same as their logical sector size. For NVMe SSDs, if it is available, the Atomic Write Unit Power Fail (AWUPF) parameter value is used.

Logical sector size
The logical sector size, also known as the operating system sector size, represents the size of the sectors exposed to the operating system and applications. It is the sector size used for reading from and writing to the storage device at the software level. The logical sector size can differ from the physical sector size. For example, an Advanced Format HDD with a physical sector size of 4096 bytes may still present a logical sector size of 512 bytes for compatibility with older systems and applications.

But from this Dell Storage with Microsoft Storage Spaces Best Practices Guide doc, it seems your new file server is composed of older hard drives:

Older hard drives had a 512B physical sector size, while newer drives have a 4 KB physical sector size and either a 512B logical sector size (512e Drive) or a 4 KB logical sector size (4Kn Drive).

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