QL-SD User Manual
18
3. File System Size in MB (Megabytes):
Choose your file system size
,
but make sure you read
and understand the “Memory Considerations” text box below beforehand. If in doubt, enter “64”
- This will give you 64 MB of disk space. Plenty of space for a QL
4. Group Size
: This is the size of a disk allocation cluster - Explained in the text box “Memory
Considerations”. If in doubt, enter “8”.
The “MakeImage.exe” program will
not
warn you if you specify Group/Filesystem sizes your QL is
not fit for (How should it know how much memory you have?) - Most probably the QL will simply
crash when trying to mount that card for the first time and memory gets tight.
Very large group sizes might look desirable at first because you would be able to use more disk
space - But beware: They could even slow down read and write operations - The operating system
can only read and write complete groups, so your QL might end up in a situation where writing a
small file will actually have to transfer much more bytes than just the file size to the card
(Remember: The recommended group size of 8 means that the driver will read and write chunks of
at least 4kb - A group size of 64 will use a minimal allocation size of 32kb!).
A simple rule of thumb might be: Simply use a Group Size of 8 for everything smaller than 128MB,
16 for everything bigger than 256MB, 32 for 512MB and up. Really large images (what would they
be good for?) should use a Group Size of 64. By the time this document is written, there is really no
stable opinion on this from the beta testers.
Unfortunately, for the moment we have no native support for creating BDI QDOS images other than
on MS Windows, but:
Linux Users please note: MakeImage.exe will happily run on Wine.
MacOS X Users please note: MakeImage.exe will happily run on Mac Parallels
Memory Considerations when creating an Image File
Any block device on the QL needs memory for various purposes, and so does the QL-
SD. Memory is used for slave blocks (temporary buffers that hold data while it is being
shoved out to the SD card) or drive maps (maps that hold information where on the SD-
card which piece of the respective file can be found).
The actual amount of memory needed depends on two parameters that need to be
given when the BDI file is created:
File system Size
and
Group Size
. File system size
sounds easy - It simply specifies the amount of storage space you want in your image
file - But it has some hidden implications: The larger you specify the File system size,
the larger your disk map is going to grow - The more QL memory will be used by the
driver for buffering the map. On an expanded QL, this should not be a problem unless
you specify a ridiculously large file system - On an unexpanded box, it is - A large one.
Group size is the size (in 512-byte-sized sectors) of an allocation block on disk. The
Summary of Contents for QL-SD
Page 1: ...QL SD User Manual...
