[vbox-dev] some suggestions to improve vhd disks

Huihong Luo huisinro at yahoo.com
Wed May 18 19:43:23 GMT 2011

Compared to vdi/vmdk, vhd disk could be 2 x slower with current implementations. For example, when I dump a 250G disk to a vhd, it taks almost 2 x time to complete compared to vmdk.
The following are some suggestions that could improve the performance, as we have implemented them in our kernel vboot driver for vhd (we didn't use any vhd code from vbox for vhd disk).
(1) allocated a bitmap to keep tracking of all blocks, if a block has a sector bitmap of all 1s, then market it inside this block bitmap. This way, no need to read the sector bitmap for a block with all sectors allocated.
(2) caching sector bitmaps. We cache 256 sector bitmaps, this saves the time to read and update sector bitmaps.
These 2 can gain quite some performance, close to vmdk/vdi, at least.
- huihong
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