Format SD (256MB) only 241MB Available ?

Fjorko · Occasional Visitor Joined: Jul 12, 2004 Posts: 41

Is this normal ? I formatted my 256MB Kingston with storage tools and it shows available only 241MB ?

Is this correct ?

tom9851 · Posted: Wed Jul 14, 2004 10:52 am Post subject:

Yep that sounds about normal for a 256mb card - also 241mb on a Lexar 256mb SD 32X card so make doesn't seem to be relevant. You will have about 120mb of useable memory on a 128mb card 8O

Not quite a full pint is it!!

Tom
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IPAQ2210/ROM-1.10/LEXAR 32X 256MB SD/Kingston ElitePro 1GB CF
TTN3-V3.03/Wired GPS-V3.03/Globalsat BT-338

tom9851 · Posted: Wed Jul 14, 2004 10:54 am Post subject:

I tell a lie - it's actually 244mb on my Lexar and 241mb on my Sandisk

Tom
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IPAQ2210/ROM-1.10/LEXAR 32X 256MB SD/Kingston ElitePro 1GB CF
TTN3-V3.03/Wired GPS-V3.03/Globalsat BT-338

Darren · Posted: Wed Jul 14, 2004 11:23 am Post subject:

This is perfectly normal for any formatted storage media. Some data is reserved for the File Allocation Table (FAT). The amount of space used is also dependent on whether you use FAT16 or FAT32 and the cluster size also has a bearing upon this.
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Darren Griffin

Fjorko · Occasional Visitor Joined: Jul 12, 2004 Posts: 41

Are these Kingstons fast SD Cards ?

raboyme · Posted: Sat Jul 17, 2004 8:36 am Post subject:

What is the difference between 16 and 32 FAT and the clusters etc.
I've posted this question a few days ago, but did not get any good answears!

timc · Posted: Sat Jul 17, 2004 6:16 pm Post subject:

raboyme

Differences between FAT16 and FAT32. Its all about the File Allocation Table (FAT) being able to address all the clusters on a Drive/Device.

Sorry for a long post, but if you stick with it, you won't have to ask again.

Some history is required first...

When the first Personal Computers were developed, they used floppy disks with a capacity of 180 kilobytes, soon followed by doublesided
diskettes of 360KB. These were the 5 1⁄4 in diskettes that you probably might remember. In order to organize data on these storage devices, the initial developers of DOS created tables to indicate which sectors belonged to which files and which sectors were vacant. This allowed the Operating System (DOS) to make maximum use of the limited space on these diskettes. Files could be saved without having a contiguous series of sectors available. The data for the files could be broken up and scattered all over the diskette in an organized manner that is now called fragmentation. However, at that time, it was nice to be able to store your
data on the same diskette as the operating system so you didn’t have to keep switching diskettes in and out during the operation of the program.

On a 360KB diskette there are 768 sectors. Thus, the table had to be big enough to identify each of the 768 sectors and allow some room for growth since the developers knew that larger diskettes were coming. Using one byte per table entry only permitted 512 possible values so that would not do. Two bytes would permit 16384 entries and would be wasteful of disk space. So the decision was made to use 12 bits (1 1⁄2 bytes) as the length of the table entries to keep track of sector usage on these first diskettes. This permitted 4096 possible values and if each entry referenced a single 512-byte sector then this FAT table could handle storage devices of up to 2 GB. However, this resulted in long tables that took time to retrieve and could not be kept in memory since memory sizes were usually less than 64 Kilobytes!

So the concept of clusters was incorporated. This increased the minimum element of storage on a device from a single sector to a fixed number of sectors, called a cluster. For those first 180KB diskettes, the cluster size was 2 or 1024 bytes. Thus if one was saving a 500 byte file, it would occupy 1024 bytes on the diskette. This seems wasteful but in reality the cluster concept works great on larger files which most of the system and user files tend to be. The table below illustrates the maximum number of clusters that can be represented by a FAT.

FAT SIZE in bits ------ Number of Clusters
12 --------------------- 4096
16 ------------------- 65536
32 --------- 4,294,967,296

Clusters...

The smallest element of addressable storage on a disk is called a sector and usually holds 512 bytes (or characters). Since most files are much larger than a sector, an even larger amount of storage should be the smallest amount allocated to any particular file. This allocated amount is called a cluster and depending on the capacity of the storage device (diskette, flash card, hard drive) the size of the cluster will vary to get optimum use of the device. On the early 360KB diskettes, the cluster size was 2 sectors (1024 bytes). Using the first 10MB hard disk, the cluster size was increased to 8 sectors (4096 bytes). Typical cluster sizes for compact flash devices today are 8K (8192 bytes) or 16K (16384 bytes). Hard disks up to 2 Gigabytes have a cluster size of 32K. The Table below illustrates the maximum size of a partition given a cluster size and a FAT type.

Cluster Size (bytes) ---- FAT 12 ---- FAT 16 ---- FAT32
1024 (1K) ------------------- 4.1MB ---- 67MB ---- 4.398TB
4096 (4K) ------------------ 16.7MB ---- 268MB ---- 17.59TB
8192 (8K) ------------------ 33.4MB ---- 536MB ---- 35.18TB
16384 (16K) --------------- 66.8MB ---- 1.07GB ---- 70.36TB
32768 (32K) ------------- 134.2MB ---- 2.14GB ---- 140.7TB

Therefore, with an 8K cluster size, a 500 byte file will take 500/8192, which is less than 1, but as you need at least one its takes 8182 (8k) bytes or 1 cluster with rest being wasted, a 1Mb file will take 1024k)/8k = 128 Clusters, very efficient.
To see a bad case, If you chose a 32k cluster size, and stored 500 bytes, you would still use up 32K, very inefficient.

So what is the big deal about FAT? Up to now, the Compact Flash storage devices or SD storage devices have used either a 12-bit or 16-bit FAT. Digital cameras and MP3 players have been designed to read and write using this table structure. With the coming of the 4GB Microdrive, the 16-bit FAT is not large enough to handle all the sectors. With 16 bits, only 65536 clusters can be be presented and with the maximum cluster size of 32K bytes, therefore only 2.14 GB can be accommodated in a single FAT. There are two solutions. Increase the FAT size to 32 bits or split the drive into multiple partitions.

For most CF/SD cards, the FAT16 should be capable of being able address the amount of storage on the card, well upto 2.1GB if formatted with 32K cluster size or 1.07GB with a 16K cluster size.

As you can see from above it is therefore possible to format a 256 Mb card with FAT16 or FAT32. It all comes down to a compromise involving how much space is available on the storage device, the typical file sizes you think you will be handling and what the device is capable of reading.

If you want to share the card between a camera/mp3 player and your PDA, you need to make sure that both devices can access a FAT32 formated drive, otherwise stick with FAT16

I hope this clears up what FAT 16 and FAT 32 is all about..

Thanks are due to a buried white paper by Dr. William F. Heybruck on the Hitachi website which I heavily reused. I started to write an article from scratch, but he can write a lot better than me. Rolling Eyes

& life is just too short

Cheers

Tim

raboyme · Posted: Sun Jul 18, 2004 1:15 pm Post subject:

Thank's Tim!

Roy

Commander · Posted: Mon Jul 19, 2004 12:12 am Post subject:

Wonderful explanation Tim, I was able to follow it too!

What it does not explain, is whether there is an "ideal" FAT/Cluster set up, where more than one is available (except as regards efficient use of space), or if different combinations are better for different applications.

Which settings would you recommend for a SD 256MB card which is used for listening to MP3 files, spoken word?

Peter

cullis · Occasional Visitor Joined: Sep 20, 2003 Posts: 6

Commander,

What to choose really depends on how applications store and access data.

FAT16 + Larger cluster Size - use for MP3 and TOMTOM maps

FAT32 + smaller cluster size - use for cards larger than 64-128MB and when storing a lot of small files.

The above is my view, but nothing beats a test.

I have a 2210 to hand, with a SD card & CF card. I will run some tests on calculating routes of about 200 Miles on a the entire UK map (single segement) with both FAT32 and 16, each with a small and large cluster size. I will also run each test at least threee times to see if any caching is being used by the apps.

The SD is a Lexar 256MB, and the CF is a Lexar 128MB.

This will take a couple of days, so please be patient

Commander · Posted: Tue Jul 20, 2004 8:47 am Post subject:

Hi Cullis

This sounds facinating, I had never thought that the way a card was formatted could affect the speed withwhich it works.

Taking that a step further, would it be true to say that the formatting will affect the amount of work which the processor has to do, and therefore might contribute to the processor overload which has been refered to in these forums as a possible cause of lock ups?

I await your results with interest. As the Lexar cards seem to be regarded as "good" the question of dodgy media should not be an issue.

Peter

cullis · Occasional Visitor Joined: Sep 20, 2003 Posts: 6

Some early results on the SD card..

I planned a route from Girton, Cambridge, to Ruthin in North Wales, a journey of 179.1 miles. I chose a cross country route to get a mixture of fast roads and slow roads in order to get some I/O to the disk. I ran each test 3 times & created an average

I formatted card:

FAT Level Cluster Key Time (sec)

16 .......... 4 ............... 17.46
16 .......... 8 ............... 16.59
16 .......... 16 ............... 16.55
16 .......... 32 ............... 16.88

32 .......... 64 ............... 16.85

These are the results so far! Numbers suggest FAT 16 with 16bit cluster key, but more to come tomorrow, (I forgot my card reader so installing the UK map is boring to say the least.

SteveW · Posted: Tue Jul 20, 2004 11:33 pm Post subject:

Interesting - I have GB maps on my 256 Lexar 32x SD card which is FAT16 with 16 kB Clusters and it took 13 seconds (tried twice) with a route distance of 183.3 miles. The route difference was probably because I put the town in and cleared the address fied which should then start from the centre of the town.

Steve

SteveW · Posted: Tue Jul 20, 2004 11:34 pm Post subject:

p.s.....time was from pressing OK to when the screen comes up with the distance.

timc · Posted: Wed Jul 21, 2004 8:26 am Post subject:

As my gps isn't connected, in the map view, I plan the route by setting departure and destination, with type set to quickest, I measure from when I click ok to the point when the map summary is displayed. The timings are taken using a digital stopwatch on my host laptop.

Tim

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