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

[Fam Zheng]

On Wed, 07/20 09:09, Paolo Bonzini wrote:
> 
> 
> 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

Hah? I'm apparently missing something. I can't see these entries:

> address@hidden:/var/tmp$ cat /tmp/sh
> touch test
> 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
> qemu-img map test
> address@hidden:/var/tmp$ sh /tmp/sh
> Offset          Length          Mapped to       File

(I'm using Fedora 24 but I've also tried RHEL 7.)

Fam

> 
> 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