[General Questions and Answers] robert__:
Indeed, that is the idea. After installing some fairly basic stuff, I only had ~1GB left which is insufficient for large apps like offline navigation.

However, you may find that the internal storage is quicker then the actual SD card, so you may want to check that you don't slow down your apps.

I installed busybox, and ran the following;

# time dd if=/dev/zero of=/mnt/sdcard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 9.128 secs (11487467 bytes/sec)
0m9.18s real 0m0.00s user 0m1.07s system
# time dd if=/dev/zero of=/mnt/sdcard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 10.297 secs (10183315 bytes/sec)
0m10.37s real 0m0.00s user 0m0.96s system
# time dd if=/dev/zero of=/mnt/sdcard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 9.370 secs (11190779 bytes/sec)
0m9.54s real 0m0.00s user 0m1.00s system
# time dd if=/dev/zero of=/mnt/extSdCard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 17.478 secs (5999404 bytes/sec)
0m17.83s real 0m0.00s user 0m1.38s system
# time dd if=/dev/zero of=/mnt/extSdCard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 11.267 secs (9306612 bytes/sec)
0m11.73s real 0m0.01s user 0m1.45s system
# time dd if=/dev/zero of=/mnt/extSdCard/testfile bs=104857600 count=1
1+0 records in
1+0 records out
104857600 bytes transferred in 12.553 secs (8353190 bytes/sec)
0m12.85s real 0m0.00s user 0m1.54s system

This times the creation of a 100MB file, three times in a row. The MicroSD card is a 32GB SanDisk Ultra MicroSDHC UHS-I card, which is rated at up to 30MB/s

read

speed.

Now clearly I don't get anywhere near that, but writes are always slower. My actual write speed is between 5.72 and 8.87MB/s in the above test, while the performance of the internal flash memory was between 9.71 and 10.95MB/s.

SanDisk has faster memory, the "Extreme Pro", which is rated up to 95MB/s read and 90MB/s write (and only available up to 16GB in capacity). But looking at the performance numbers I suspect that this will not perform better as I think the bottleneck is the phone itself, and that it does not support the faster UHS speed classes and is limited to Class 10.

---------- Post added at 04:40 PM ---------- Previous post was at 04:25 PM ----------

And if anyone was wondering about read speeds of the same 100MB file

Internal memory

# time dd if=/mnt/sdcard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 3.278 secs (31988285 bytes/sec)
0m3.35s real 0m0.20s user 0m1.05s system
# time dd if=/mnt/sdcard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 2.341 secs (44791798 bytes/sec)
0m2.45s real 0m0.30s user 0m0.92s system
# time dd if=/mnt/sdcard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 2.249 secs (46624099 bytes/sec)
0m2.34s real 0m0.14s user 0m1.04s system

So here I'm getting a read speed of between 30.5 and 44.46MB/s

32GB SanDisk Ultra MicroSDHC UHF-I

# time dd if=/mnt/extSdCard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 1.005 secs (104335920 bytes/sec)
0m1.03s real 0m0.15s user 0m0.85s system
# time dd if=/mnt/extSdCard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 0.996 secs (105278714 bytes/sec)
0m1.01s real 0m0.18s user 0m0.81s system
# time dd if=/mnt/extSdCard/testfile of=/dev/null
204800+0 records in
204800+0 records out
104857600 bytes transferred in 1.004 secs (104439840 bytes/sec)
0m1.01s real 0m0.16s user 0m0.84s system

These numbers are incredibly quick, between 99.5 and 100.4MB/s. Which either means that the operation is being cached in memory, or the phone does support UHS-I.