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Hard Drive Knowledge: Blocks vs. Sectors

11/28/2013

21 Comments

 
A sector is a physical spot on a formatted disk that holds information. When a disk is formatted, tracks are defined (concentric rings from inside to the outside of the disk platter. Each track is divided into a slice, which is a sector. On hard drives and floppies, each sector can hold 512 bytes of data.
A block, on the other hand, is a group of sectors that the operating system can address (point to). A block might be one sector, or it might be several sectors (2,4,8, or even 16). The bigger the drive, the more sectors that a block will hold.
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So why are there blocks. Why doesn't the operating system just point straight to the sectors? Because there are limits to the number of blocks, or drive addresses, that an operating system can address. By defining a block as several sectors, an OS can work with bigger hard drives without increasing the number of block addresses. For example, PC DOS (earlier versions at least) could only address 65,536 blocks (64K), and each block could could only be a single sector. Thus, the largest size a disk volume could be was 32mb (64K * 512K). (Earlier versions of the Mac OS had a 16mb volume limit for similar reasons). If you increase the size of a block to, say, 4K, that same version of DOS can now work with volumes as large as 256MB (64K addresses * 4K blocks).
With current versions of the OS's, the formatting software will look at the size of the drive, and figure out the smallest number of sectors that need to be in a block in order to be able to use the entire drive. So, when you format a floppy disk, the block size will be one sector. When you format a 230MB drive, for example, the block size is 8 sectors (4K).  Why does this matter?


True or False: When a file is copied from a hard drive to a floppy, it will usually take up less space than it takes up on a hard drive.
TRUE. Although the size of the file will be the same, fewer sectors will be used to store the file. Conversely, when a file is copied from a floppy to a hard drive, it will usually take up more disk space. When files are stored on a disk, they always use up a whole number of blocks. Any unneeded space at the end of a block is unused and wasted. For example, say your hard drive has a block size of 4K, and you have a file that is 4.5K. This requires 8K to store on your hard drive (2 whole blocks), but only 4.5K on a floppy (9 floppy-size blocks). 
Miscellaneous info...If you tend to store lots of small files on your hard drive (like running Windows and Windows apps), the blocks used to store all those little files can have a
lot of wasted space in them. Likewise, compressing lots of little files may not save as much space on a big hard drive with a big block size. If the block size is 4K, and you compress a 3K file, the file will be compressed, but it will still use 4K of disk space. If you do a Get Info on a file on a Mac, the Size info will say something like '12K on disk, 8320 bytes used'. The 12K is the amount of disk space used, based on the block size. Thus, if the block size of your drive is 4K, this number will always be in increments of 4K. 8320 bytes is the actual size of the file. Note that you have to go to Get Info to see the actual size of the file. This number does not show up in View by Name 

-by Help at Filemaker
21 Comments
VriKab
2/25/2015 05:55:00 am

Nice article. Just a minor correction:
This -> 32mb (64K * 512<b>K</b>)
should be -> 32mb (64K * 512)

Reply
Gurpreet singh
9/11/2015 07:59:37 am

Why operating system can address only to limited number of blocks ??

Reply
PacMan
8/23/2016 10:50:23 am

Please replace all occurrences of 'block' in the above post by 'clusters'.
A LBA (Logical Block Addressing) is normally 512 bytes (for harddisks it is always 512 bytes). The large harddisks being manufactured today may have sectors of 4096 bytes each; this means the number of blocks per sector is 8!
A cluster, on the other hand, is often two sectors, so if the harddisk or floppy disk has a 512 byte sector size, the cluster size is 1024 bytes (1K); this is often the case with FAT-formatted floppy disks.
Other file systems than FAT/FAT16/FAT32 most likely do not use clusters at all.
-So clusters are usually a FAT term.

Please correct the above mistakes.

Reply
Chrome link
12/16/2016 05:29:49 pm

Awesome explanation my good man!

Reply
Jemel
6/17/2017 10:29:33 am

Great breakdown. Thanks for the concise info and great examples.

Reply
Tounsi
8/5/2017 05:56:14 am

nice and clear

Reply
RAJA V S
3/3/2018 09:30:51 am

Thank you for such a simple and crisp explanation. I am into IT for many years, still I struggle to understand the concept of block storage. Now I know better after reading your blog.

Reply
sneha
11/27/2018 04:05:45 am

This article is very useful.thanks to share with us

Reply
Luke
4/2/2019 04:26:11 am

Thank you!

Reply
Nathan
5/21/2019 04:51:35 am

Thanks! Very concise and informative, and the examples were illuminating.

Reply
BenG
2/4/2020 09:43:51 am

" If you increase the size of a block to, say, 4K, that same version of DOS can now work with volumes as large as 256MB (64K addresses * 4K blocks)."
I think what you mean is if the sector size is increased to 4K, then with the same 1 block=1 sector limitation, a larger disk can be used. Or perhaps you meant that this limitation has been increased to '1 block=8 sectors', and therefore 4k in size?

Reply
kra doros
3/11/2020 07:07:20 am

Very usefull artcle. But there are mistakes. 32MBis not 64K * 512K.
32MB is 64K * 512 Bytes (~32000000 Bytes)

Reply
abc
4/27/2020 11:09:36 am

Thanks! Well explained!

Reply
data recovery dubai link
7/13/2020 09:42:46 pm

We know that hard drive is very important part of operating system. All data store in hard drive. But if the system crash or hard drive problem occur then we lost our important data. We also can data recovery through service center.

Reply
Himanshu Sharma
12/2/2020 03:40:44 am

beautifully explained

Reply
gijom
12/3/2020 03:47:43 am

Just to mention a typo:
"thus, the largest size a disk volume could be was 32mb (64K * 512K)"
->
"thus, the largest size a disk volume could be was 32mb (64K * 512 bytes)"

Thanks for this helpful post !

Reply
cybes
12/6/2020 08:01:35 am

Thank you so much, I'll be giving reference to this website in my IOPS page. Hope that wouldn't be a problem. Very well written and explained especially those miscellaneous info.
Thank you

Reply
danny
4/15/2021 04:53:12 am

Thank you very much sir

Reply
ameno
11/24/2021 08:43:25 pm

nice article but please change the font color

Reply
joe Schmoe
12/6/2021 09:03:56 am

You mean 32GB maximum disk size, right?

Reply
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2/3/2022 03:56:36 am

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