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Re: [Qemu-devel] [PATCH v5 13/14] nbd: Implement NBD_CMD_WRITE_ZEROES on


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v5 13/14] nbd: Implement NBD_CMD_WRITE_ZEROES on server
Date: Wed, 20 Jul 2016 09:09:10 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1


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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