Re: [Qemu-discuss] qcow2 performanceimprove

[yang.bin18]

>  > > > > If there is no backing file or snapshot you still need to fill

> > > > > the cluster with zeroes, and that's going to be slower with

>  > > > > larger clusters.

> > > > If not fill zeroes and only write guest data ,what`s wrong could

> > > > happen ?

> > > The following could happen:

> > > 1) Guest reads at offset  [0,   4k] -> there's only zeroes

> > > 2) Guest writes at offset [8k, 16k]

> > > 3) Guest reads at offset  [0,   4k] -> there's something else now




> > Why could guest read the area at offset [0, 4k] has not be writen

> > yet ?




> Jakob already gave you some answers, but here's a simple one: because

> it might have already been written.




> If the guest wrote zeroes to [0, 1M] you can't generally assume that

> there's an allocated 1MB cluster on the qcow2 file filled with zeroes.




>  - QEMU can detect that the guest tried to write zeroes and decide to

>    leave the cluster unallocated (see for example the "detect_zeroes"

>    option, or the "WRITE SAME" SCSI command).

>  - The qcow2 file could have been converted at some point, and

>    zero-filled clusters could have been deallocated for efficiency.




> Berto




Thank for every replay,and I understand completely . 

Then I look forward to using the   “subcluster allocation ”feature  as early as 
possible.





Yang.bin