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What are the performance differences between phone's internal memory, the internal SD card, and the external SD card? I wonder if that counts when deciding where to store apps. What technologies are used for building a phone's internal memory? How about the internal SD card, if it is not really an SD card at all? How do they compare, in general, to SD card speed classes 4, 6 and 10?

Note that I am not asking about the filesystems, as a deleted answer focused on, since performance comparison on this aspect would be totally ruined by just switching the SD card from FAT32 to something better (which I think the industry should have done since a long time anyway).

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    I bet there wouldn't have to be major differences between using the internal memory and an internal non extractable external storage. And mainly it's the same. You normally put things in the internal memory when you want data isolation on non rooted devices. Why bother? When programming applications just use the internal data directory (and every sub folder you would want to create in here) for private programs and data storage, and the external storage for media and such data non bound to the apk, or different apk's collaboration.
    – 43488
    Commented Dec 10, 2013 at 13:05

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For what's worth, I had a class 4 SD card. Running games whose data was on that external SD was painfully slow, during loading and during gameplay, with frequent mini-freezes. After upgrading to a class 10 card, I find no difference in running games from the internal and the external storage.

Please don't draw any conclusions from these highly rigorous scientific experiments.

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I'll give some numbers collected while testing my Samsung Galaxy SII GT-I9100 with 16 GB internal memory (11 GB available for user files) and 64 GB SanDisk MicroSDXC memory card (UHC-I SDSDQU-064G-U46A):

------------------------------------------------------------
|        |     Internal memory    |     External card      |
| File   |------------------------|-------------------------
| size   |    Read    |   Write   |    Read    |   Write   |
----------------------------------|-------------------------
| 300 MB | 33,34 MB/s | 5,35 MB/s | 11,44 MB/s | 8,52 MB/s |
------------------------------------------------------------

Larger files make the write speeds to lower slightly (ca. 7 MB/s for ext card and almost no change for internal memory).

So, in this very case, on this phone and a memory card of exactly this type, internal memory has better reading performance, but there is a catch in it: Flash memory wears out the more the often you write to it, so both the internal memory and the card may become faulty some day. And when this happens a memory card is easier to replace. To replace internal memory you should go to repairing service.

What about flash memory speeds it is all about your feeling - do you feel that using the slower external card fits your needs or you notice a slowdown. I've set up camera app to write pictures and movies to external card and do not feel any discomfort.

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