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

Fernando Rodriguez <frodriguez.developer@×××××××.com> [15-08-08 05:42]:
> On Friday, August 07, 2015 7:01:29 PM Mick wrote:
> > On Friday 07 Aug 2015 04:27:15 Fernando Rodriguez wrote:
> > > 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).
> >
> > Excellent information Fernando, thank you!
> >
> > So there is no tool for me to use to read the AU/RU on the chip?
> >
> >
>
> The RU can be calculated from the card size and speed class, that's the number
> printed on the card label inside the C. I don't know of any tools to get the
> AU and it looks like it's not exported to userspace so any such tool would
> have to guess it. However, if you want to hack your kernel all it takes is one
> line on /usr/src/linux/drivers/mmc/core/debugfs.c. Add the following towards
> the end of mmc_ios_show before the return statement:
>
> seq_printf(s, "au (sectors):\t%u\n", host->card->ssr.au);
>
> then you can do:
>
> cat /sys/kernel/debug/mmc0/ios
>
> Multiply the value by 512 to get the size in bytes. It can be up to 64MB.
>
>
>
> --
> Fernando Rodriguez
>


Hi Fernando,

I looked into the kernel ... but ... hmmmm ...
for me it seems this is only for onboard mmc flash...
which is often found on embedded linux boards.
Or am I wrong?
I am using a micro sdcard in a sdcard-reader plugged
into an USB port of my Linux PC.
(Later it will be inserted into the sdcard slot of
my tablet PC...)
I am asking just to prevent to make things screwed up.

Have a nice sunday! :)
Best regards,
Meino