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I've been looking at logical and physical block sizes for disks...

But I'm new to SSDs, and been reading (see below) that SSDs don't have physical sectors they have physical pages... and those pages get "logically" associated to what is called a block on SSD.

SSD layout In contrast to the hard disk, a SSD consists of semiconductor memory building blocks, it contains no mechanical parts. The smallest unit of an SSD is a page, which is composed of several memory cells, and is usually 4 KB in size. Several pages on the SSD are summarized to a block. A block is the smallest unit of access on a SSD. Currently, 128 pages are mostly combined into one block; therefore, a block contains 512 KB.

Questions:

  • Are SSDs made in other page/block sizes other then this (4KB/512KB)?
  • If the SSDs "block size" comes in 512KB
    • can I logically address(4KB) -> physical(4KB)?
    • or even, logically(512KB) -> physical(512KB(128 * 4KB))?
Jordan Davis
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1 Answers1

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Currently, all IDE/SATA hard drives expose either 512B or 4KB sized blocks (depending on make and model) for read/write operations. Those are the only two options available (shame, because I can imagine other LBA sizes being very advantageous).

When an OS reads/writes to a hard drive, it has to manage the difference between the file system's sector size and the hard drive's LBA size. For a 512B hard drive, writing a 4K NTFS sector requires 8 x 512B writes. You can see how a 4K drive might perform better given that the same operation would only take 1 x 4K write.

The way SSDs do things internally varies by make and model. Page sizes can be different.

misha256
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  • I don't know how I didn't see those sizes on that wikipedia page... – Jordan Davis Sep 22 '15 at 00:18
  • what is (+128) for? `64 pages of 4,096+128 bytes each for a block size of 256 KiB` – Jordan Davis Sep 22 '15 at 00:19
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    @JordanDavis From the same wikipedia article *"associated with each page are a few bytes (typically 1/32 of the data size) that can be used for storage of an error correcting code (ECC)"*. That's what the +128 is for ;-) – misha256 Sep 22 '15 at 00:29