Use Image File (1) - Formats & Advanced Options
Because of the volatility of RAM, data held on ramdisks will be lost when power is off. In some cases, users want ramdisks to keep data permenantly, as does a physical hard disk. The VSuite Ramdisk provides the Image File Support feature which is able to save data to an image file on the physical disk and later load data from this image file. Thus, users can continue on their data after system reboot.
Image File Formats
VSuite Ramdisk supports three kinds of image files: Direct Mirror, Smart Mirror and Compressed Mirror.
The Direct Mirror is a full mirror of the ramdisk. Its file size is same as the ramdisk's size. For example, if a ramdisk's size is 2GB, the Direct Mirror for this ramdisk is also 2GB even if the ramdisk is empty.
The Smart Mirror is a compact mirror of the ramdisk. It rearranges the data layout and only stores the valid data. Therefore its file size is normally much smaller than the Direct Mirror's. For example, the Smart Mirror for the empty ramdisk of 2GB size may only be 32MB.
The shortage of the Smart Mirror is that it costs minor performance of data access.
The Compressed Mirror is a legacy format and is supported for backward compatibility. It is a compressed mirror of the ramdisk. Different compression levels are provided. Higher compression level will lead to smaller image file size, but it takes more time. It may be time comsuming to compress data held on a ramdisk of big disk size. Default level is "Least Compression" level, which has the least compression ratio but the fastest process.
1. The total time spent in saving image file is the sum of compression time and disk IO time roughly. Lower compression level requires lower compression time, but leads to bigger compressed file which requires more time to access physcial hard disk. User may balance this two factors.
2. Compression time is related to the data held on the ramdisk. Time varies if data varies.
VSuite Ramdisk provides advanced options for the Image File Support feature: Quick Save Mode and Quick Load Mode.
The Quick Save Mode makes the ramdisk only save new or updated data and skip unchanged data, thus avoiding writing all data every time, which in turn, reduces a lot of time of writing image file. Although it costs minor performance of data access, it's well worth it.
1. For the Direct Mirror, it may still take some time to save the image file at the first time because the Direct Mirror needs time to build the initial data.
2. The Compressed Mirror doesn't support Quick Save Mode.
Normally the whole image file is loaded during system startup, which may prolong the startup process especially when the image file is very large. The Quick Load Mode makes the ramdisk only load requested blocks of the image file if these blocks haven't been loaded yet. Thus, it minimises the influence on the system startup process. In addition, it may reduce the number of read access to the physical disk because normally it doesn't need to read the full image file.
The Quick Load Mode also brings minor performance decrement. Besides, sometimes users may feel a lag when running applications or requesting data from the ramdisk at the first time, because the ramdisk needs to load data blocks from the image file for the first run.
If users don't care the sartup time or the image file size is less than 1GB, it is recommended to disable this mode to achieve the best performance.