My program needs to read chunks from a huge binary file with random access. I have got a list of offsets and lengths which may have several thousand entries. The user selects an entry and the program seeks to the offset and reads length bytes.

The program internally uses a TMemoryStream to store and process the chunks read from the file. Reading the data is done via a TFileStream like this:

FileStream.Position := Offset;
MemoryStream.CopyFrom(FileStream, Size);

This works fine but unfortunately it becomes increasingly slower as the files get larger. The file size starts at a few megabytes but frequently reaches several tens of gigabytes. The chunks read are around 100 kbytes in size.

The file's content is only read by my program. It is the only program accessing the file at the time. Also the files are stored locally so this is not a network issue.

I am using Delphi 2007 on a Windows XP box.

What can I do to speed up this file access?