Live Long and Prosper: The Logical Page

Computers are all about abstraction. In the early days of computing you had to write assembly code to get your hardware to do anything. Programming languages like C and C++ created a layer of abstraction between the programmer and the hardware, simplifying the development process. The key word there is simplification. You can be more efficient writing directly for the hardware, but it’s far simpler (and much more manageable) to write high level code and let a compiler optimize it.

The same principles apply within SSDs.

The smallest writable location in NAND flash is a page; that doesn’t mean that it’s the largest size a controller can choose to write. Today I’d like to introduce the concept of a logical page, an abstraction of a physical page in NAND flash.

Confused? Let’s start with a (hopefully, I'm no artist) helpful diagram:

On one side of the fence we have how the software views storage: as a long list of logical block addresses. It’s a bit more complicated than that since a traditional hard drive is faster at certain LBAs than others but to keep things simple we’ll ignore that.

On the other side we have how NAND flash stores data, in groups of cells called pages. These days a 4KB page size is common.

In reality there’s no fence that separates the two, rather a lot of logic, several busses and eventually the SSD controller. The latter determines how the LBAs map to the NAND flash pages.

The most straightforward way for the controller to write to flash is by writing in pages. In that case the logical page size would equal the physical page size.

Unfortunately, there’s a huge downside to this approach: tracking overhead. If your logical page size is 4KB then an 80GB drive will have no less than twenty million logical pages to keep track of (20,971,520 to be exact). You need a fast controller to sort through and deal with that many pages, a lot of storage to keep tables in and larger caches/buffers.

The benefit of this approach however is very high 4KB write performance. If the majority of your writes are 4KB in size, this approach will yield the best performance.

If you don’t have the expertise, time or support structure to make a big honkin controller that can handle page level mapping, you go to a larger logical page size. One such example would involve making your logical page equal to an erase block (128 x 4KB pages). This significantly reduces the number of pages you need to track and optimize around; instead of 20.9 million entries, you now have approximately 163 thousand. All of your controller’s internal structures shrink in size and you don’t need as powerful of a microprocessor inside the controller.

The benefit of this approach is very high large file sequential write performance. If you’re streaming large chunks of data, having big logical pages will be optimal. You’ll find that most flash controllers that come from the digital camera space are optimized for this sort of access pattern where you’re writing 2MB - 12MB images all the time.

Unfortunately, the sequential write performance comes at the expense of poor small file write speed. Remember that writing to MLC NAND flash already takes 3x as long as reading, but writing small files when your controller needs large ones worsens the penalty. If you want to write an 8KB file, the controller will need to write 512KB (in this case) of data since that’s the smallest size it knows to write. Write amplification goes up considerably.

Remember the first OCZ Vertex drive based on the Indilinx Barefoot controller? Its logical page size was equal to a 512KB block. OCZ asked for a firmware that enabled page level mapping and Indilinx responded. The result was much improved 4KB write performance:

Iometer 4KB Random Writes, IOqueue=1, 8GB sector space Logical Block Size = 128 pages Logical Block Size = 1 Page
Pre-Release OCZ Vertex 0.08 MB/s 8.2 MB/s

A Quick Flash Refresher The Cleaning Lady and Write Amplification
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  • paulgj - Saturday, October 09, 2010 - link

    Well I was curious about the flash in my Agility 60GB so I opened it up and noted a different Intel part number - mine consisted of 8 x 29F64G08CAMDB chips whereas the pic above shows the 29F64G08FAMCI. I wonder what the difference is?

    -Paul
    Reply
  • Bonesdad - Sunday, October 10, 2010 - link

    Been over a year since this article was published...still very relevant. Any plans to update it with the latest products/drivers/firmware? There have been some significant updates, and it would be good to at least have updated comparisons.

    Well done, more more more!
    Reply
  • hescominsoon - Thursday, February 17, 2011 - link

    Excellent article but you left out sandforce. I'm curious if this was an oversight or a purposeful moission. Reply
  • PHT - Friday, September 28, 2012 - link

    This article is fantastic, the best I ever read about SSD.
    Any follow up with new SATA III drives and new controllers like SandForce, new Indilinx etc.?
    I will be glad to see it.

    My Best
    Zygmunt
    Reply
  • lucasgonz - Wednesday, October 16, 2013 - link

    Hello everyone.
    This post is quite old but I hope someone can answer.
    I am concerned about the life of my ssd (sandisk extreme 240). I performed partitions ignoring the issue of the level of wear and partitions. I have it for one year ago with a 30gb partition and one with 200GB. I wanted to use large drive for data but I did not have time for that and just use the first 30gb partition . My question is if the ssd may be damaged by using only a little segment. DiskInfo shows 10tb reading 18 tb and writing.
    sorry my poor English.
    Thanks for any help.
    Reply
  • Ojaswin Singh - Monday, January 13, 2014 - link

    Hey,This is the most informative article i have ever read.Can You Please clear Out Some Of my Doubts:-
    1.Does Playing Video Games or Running Programs add to Writing on the SSD
    2.Is 1 Write Cycle=Filling 120GB of SSD once
    3.I really write on my HDD a lot(Seriusly a Lot) So how much life cycle can i expect from Samsung 840 SSD(Neither Pro nor EVO) I mean for how much time can i expect it to be writable
    Please Help me cause i want the speeds of SSD but i want it to last for me too
    Thanks,
    Ojaswin
    Reply

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