Re: [gentoo-user] [OT]: Optimal formatting a SDcard (64GB) with partions of diffent sizes and filesystems?

[Fernando Rodriguez]

On Thursday, August 06, 2015 6:18:59 PM Meino.Cramer@×××.de wrote:
> Hi,
>
> for my tablet PC I used a used 32GB FAT32 formatted SDcard. The
> formatting was already done by the manufacturer.
> Then I screwed it up and had to do the partioning and formatting
> myself again. "No big deal", I thought -- and was wrong.
> Yes, the "thing" I got could be read and written. But it was
> DAMN slow in comparison to the original formatting.
>
> I googled and found a description, which described exactly,
> what I wanted: An optimal formatting for one big FAT32 partion.
> I did it again ;) and: TADA! The speed was back.
> LINK:http://zero1-st.blogspot.de/2012/05/formatting-fat32-volumes-larger-than.html
>
> Now I need the something identical but explained in a way
> that it can be successfully applied to any partion layout
> and any SDcard size.
> Currently the new SDcard has 64GB (yes, the tablet eats that size
> well :) and needs at least two partions: One FAT32 and one ext4.
> May be that I need a different layout later.
>
> To what aspect and "logic" do I have to keep my eyes on, when
> it comes partioning/formatting any SDcard size with any partion
> layout and any filesystem?
>
> Thank you very much in advance for any help!
> Best regards,
> Meino

I wrote a long reply to this and it appears to have been swallowed by 
/dev/null.

SD cards don't have 128K blocks. Except for the very early ones (standard 
capacity), they are divided in allocation units (AU) that are 1MB to 4MB for 
SDHC and even larger for SDXC. The only way to get that value is by reading a 
register in the card (so you can't do it in usermode on linux).

The AUs are divided into Recording Units (RUs). The size of these can be 
deduced from the card speed class (that's the number inside the C on the 
label), and the card capacity. For class 2 and 4 if the card is less than 1GB 
it's 16KB, otherwise it's 32KB. For class 6 it is 64KB, and for class 10 it's 
512KB.

After an AU is erased you can write to any of the free RUs in any order in 
blocks of 512 bytes sequentially (the block size is configurable by the driver 
but 512 is the most common). But if you write to a nonfree RU then all non-
free RU get copied to a new AU. So the performance hit depends on how many 
non-free RUs are in the AU when this happens.

So to get the best performance you need to align the first FAT cluster on an AU 
boundary and that the RUs used by the reserved sectors after the FAT are free. 
This is not so easy from usermode because you can't get the AU size and you 
can't erase the AU to make sure reserved sectors are free. The Windows 7 and 
later format utility will do it if you don't partition the card. The next best 
thing is to align it to an RU which should be pretty easy.

You could guess the AU size by writting blocks of RU size from the start of 
the card and timing it. Every time you hit the AU boundary there will be a 
longer delay.

For more details see the SD specification (chapter 4.13).

https://www.sdcard.org/downloads/pls/

They also have formatter tools for Windows and OSX. I tried the Windows 
version years ago but had problems with it (can't remember what).

-- 
Fernando Rodriguez