On 20/07/2016 06:37, Fam Zheng wrote:
> Yes, you are right about this, I was confused because "qemu-img map" does not
> report this allocation state after zero write. (No idea why SEEK_DATA doesn't
> hit the fallocate'ed area.)
Apparently it's because it's zeroed.
$ fallocate -z -o 10485760 -l 10485760 test
$ fallocate -p -o 49152000 -l 10485760 test
$ fallocate -o 49152000 -l 10485760 test
$ fallocate -p -o 65536000 -l 10485760 test
Now we have:
- a zero area at 10240K..20480K
- an hole+allocated area at 48000K..59240K
- a hole at 64000K..74240K
$ qemu-img map test
Offset Length Mapped to File
0 0xa00000 0 test << ends at 10240K
0x1400000 0x1ae0000 0x1400000 test << ends at 48000K
0x38e0000 0x5a0000 0x38e0000 test << ends at 64000K
0x4880000 0x1b80000 0x4880000 test
So "qemu-img map" hides both zeroed and hole areas. With the JSON format
we get more information:
$ qemu-img map --output=json test
[{ "start": 0, "length": 10485760, "depth": 0, "zero": false, "data": true,
"offset": 0},
{ "start": 10485760, "length": 10485760, "depth": 0, "zero": true, "data":
false, "offset": 10485760},
{ "start": 20971520, "length": 28180480, "depth": 0, "zero": false, "data":
true, "offset": 20971520},
{ "start": 49152000, "length": 10485760, "depth": 0, "zero": true, "data":
false, "offset": 49152000},
{ "start": 59637760, "length": 5898240, "depth": 0, "zero": false, "data":
true, "offset": 59637760},
{ "start": 65536000, "length": 10485760, "depth": 0, "zero": true, "data":
false, "offset": 65536000},
{ "start": 76021760, "length": 28835840, "depth": 0, "zero": false, "data":
true, "offset": 76021760}]
Both zeroed and holes are reported as "zero": true, "data": false. This
limitation stems from the fact that we cannot use FIEMAP.
Paolo
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