Disambiguation: logical / physical block

filesystemshard drive

The terminology BLOCK confused me as I am doing some research about disk technology these days.

My Former Understanding

I understand BLOCK generally means a batch of data seen as a whole I/O unit, as described in the wiki page: Block. What I thought a logical block, or a filesystem block refers to is the minimal I/O unit seen as a whole used by specific filesystem, in order to, as the essence of any batch operation, reduce the overheads brought by reading or writing one sector one time. And a physical block is exactly the synonym of a disk sector. This article from Oracle confirmed my understanding.

What's more, I believe the word CLUSTER is just filesystem block in Microsoft's fashion, as suggested by this thread on Reddit.

What Confused Me

The wiki page: Logical Block Addressing I recently came across caused some confusion.

As the substitute for the CHS addressing, it seems that LBA should handle the same task as CHS did: give addresses to sectors, as suggested in this article. But I sense the reluctance of the LBA wiki page to use the word "sector", as well as from its title "Logical Block Addressing".

So, is LBA just a fancy word which actually concerns addressing disk sectors?

Or does LBA-compliant disks really understand the concept of filesystem/logical block, and is capable of making block-level I/O, thus hide the existence of "sector" from operating system?

Also, please correct me if any prerequisite of my question is misunderstood. Thanks.

Best Answer

I understand BLOCK generally means a batch of data seen as a whole I/O unit

The use of the term "block" is widespread in computing, and not restricted to I/O.

What I thought a logical block, or a filesystem block refers to is the minimal I/O unit seen as a whole used by specific filesystem, in order to, as the essence of any batch operation, reduce the overheads brought by reading or writing one sector one time.

IMO it would be careless to make sweeping definitions that encompass all filesystems. Be aware that the I/O blocksize for file data could be different from filesystem metadata. E.G. writes to a file could be consolidated into 4KB (or larger) blocks, but the filesystem journal may need to be written more often (with a smaller block) to ensure data retention.

"Batch operation" is old jargon, and you're using the term in a nonsensical manner.

And a physical block is exactly the synonym of a disk sector.

Only in the context of disk drives.
Magnetic tape requires I/O to be performed in physical blocks, but there is no concept of sectors with tape.

What's more, I believe the word CLUSTER is just filesystem block in Microsoft's fashion, as suggested by this thread on Reddit.

A "cluster" is a unit of allocation in MS filesystems.
Whether I/O is always performed in that same blocksize is questionable. E.G. When the cluster size is 64KB, and the entire file is just 128 bytes, is the filesystem going to write 128 sectors or optimize the I/O to just one sector?

So, is LBA just a fancy word which actually concerns addressing disk sectors?

Essentially yes (for legacy 512-byte sectors).

The integrated controller of the modern disk drive performs the mapping of LBA to physical sector. The actual cylinder, head, and sector that maps to a particular LBA is known only to the drive so that any type of zone-bit recording and relocation for bad sectors can be implemented by the disk drive.

With Advanced Format 512e HDDs that use 4096-byte sectors and a 512-byte transfer size, the term LBA is truly accurate: the address is not of a physical sector, but rather for a logical block consisting of an eighth of a sector.

Or does LBA-compliant disks really understand the concept of filesystem/logical block,

I'm not sure what you mean by "filesystem/logical block", but the answer is probably no.
It's simply a storage device with no concept of organizing the raw data it is storing.

See What kind of api does a sata hard-drive expose?

and is capable of making block-level I/O, thus hide the existence of "sector" from operating system?

The concept of sector (or physical block) cannot be eliminated, simply because that is the minimum unit of I/O. The lowest-levels of the OS (i.e. the device drivers) will always be cognizant of the hardware attributes. But each abstraction layer of the OS will try to obscure those details. So when you read a file, you may not know if it was retrieved from a HDD or DVD or over a network.

FWIW disk controllers (even old ones that used CHS addressing) can perform multi-sector read or write operations, e.g. perform a read of N sequential sectors.

Related Solutions

Do hard disk drives turn on 512e (512byte emulation of 4k sectors) as-needed depending on host controller

I've seen a few 4Kn SATA drives working in the hard drive validation industry, but I wasn't aware they were shipping any to customers, due to the limited hardware and software support for them and lack of customer demand.

There are two competing mappings for physical vs. logical sector sizes. The Long Logical Sector feature allows a device to have logical sectors longer than 512B (e.g. 4Kn), and the Long Physical Sector feature allows a device to have multiple logical sectors per physical sector (e.g. 512e), though they are not necessarily mutually exclusive. If the manufacturer is claiming the drive is 4Kn, the older controller may providing an emulation layer, like read-modify-write, for older applications and hardware, as older systems simply don't support the 4Kn format.

The sg_sat_identify command from the sg3_utils package would probably give you the information you need (check words 106-108 for Physical sector size/logical sector size information).

It doesn't look like SATA drives can support emulation on-demand, per se, but if the drive is indeed using 512e, instead of 4Kn, performance optimization is maximized by the driver (or possibly USB SATA controller) limiting reads/writes to 4K boundaries, e.g. having the transfer start on an LBA where the lower 3 bits are 0, and end on an LBA where the lower 3 bits are 1:

start_lba & 0x3 == 0
end_lba & 0x3 == 1

So, basically, there's no "switch" that can tell the drive to start or stop emulation. From an end-user perspective, your best bet is probably to make sure your drives are using the best file system for whichever mapping your drive uses and to make sure it's aligned correctly. The following site gives a pretty good breakdown of the common file systems and their stats on performance vs. block alignment on different file systems: http://www.ibm.com/developerworks/library/l-linux-on-4kb-sector-disks/index.html

Are the size of a memory page and the size of a file system cluster always the same

Always? No. Often? Yes, which is of course convenient. (Notice no claim for "usually".)

For example, with Windows:

just like in Win32, the x64 page size is 4KB

And for NTFS, the default cluster size is 4KB for disks up to 16TB. But (1) that's just the default; (2) for really large disks, the default is larger; and (3) there are other file systems.