SSD alignment Why is it good?

[silicon]

In another thread the issue was SSD alignment which I never knew about. Following some guidelines I was able to bring my SSD into correct alignment and it did make a noticeable improvement in the boot up speed. My SSD is a Sandisk extreme 480Gb and from most of the reviews it seems to be a quite acceptable model. My question is this: Why would alignment of the SSD make a difference in the speed?

 

[Cerb]

Flash pages sizes are typically 8KB, these days, with some 12KB and 16KB.

Modern HDDs and SSDs use native sector sizes of 4KB. Historically, it was 512B, or 1/8 of a new one. They translate this into 4KB accesses, internally.

File system blocks are 4KB (you can technically change it, but the correct value is 4KB, do that a block can be neatly mapped to a memory page).

Historically, the first partition began on sector 63 (beginning from 0).

So, a 4KB file system block access starting on sector 63, or any 4KB chunk aligned to that, needs to read the last 512B of one 4KB sector, plus all but the last 512B of the next. All accesses thus read or write at least one more sector than intended. This can also mean the SSD may have to read more than one page, if on a boundary, or even go to different blocks, dies, or whole channels, all for 512B.

If the partition starts on sector 64, 4KB accesses are just 4KB accesses. Some SSDs could be sensitive to the specific alignment used, in the past. Now, that's uncommon, but 1MB is the common alignment point. Nobody worries about wasting <1MB out of hundreds of GB, and it works for basically every SSD, SD card, CF card, etc..

Some SSDs in the past could be set to shift accesses to correct for it, and some internally detected it and handled it well. Now, they expect you to run a new OS, where every new partition get aligned to at least 1MB.

 

[WT]

Nicely done, Cerb !! That covers all of it.

A+ for you, and a gold star too !

 

[tidy]

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[Cerb]

I'm still confused about alignment when it comes to imaging and cloning (in my case with Macrium Reflect).

If I clone from one SSD to another identical SSD, I'd assume that alignment would be retained. What if I clone from one SSD to another SSD that is a different size or model? Would it still be aligned?

What if I take an image of my aligned SSD and then restore it back again? I'm guessing that will be fine, but what if I secure erase the SSD before restoring the image - will it still be aligned then?

Yes, though the software's particular logic would come into play if cloning to a smaller drive. Secure erasing has nothing to do with it. Alignment information is effectively "contained" in the first sector, which has the partition table, if it's MBR (if GPT, it's more complicated, but still in the partition table).

Alignment is not implemented as a device-specific thing. It was generally decided that starting partitions at 1MB would be sufficiently good for drives sensitive to alignment (including HDDs), and that storage devices are typically big enough that, even on drives where it's not needed, nobody will miss the space. Any power of two alignment from 1MB down to 512B that may be needed is effectively handled with a partition alignment of 1MB.

So, if you first partitioned a blank 500GB HDD in Windows 7+, then cloned it to a 512GB SSD, with a literal copy, it would be aligned correctly, because Windows is going to start the first partition at 1MB, unless you tell it otherwise.