Creating COW file takes a long time

[yito24]

I found the cause why it take a long time to take a snapshot.
Related issues
#383

Ver 0.11.7 and later, it takes a long time to create a COW file that is 10% of the volume.
There is a problem with the behavior of creating COW files.
Please fix this issue so that COW files can be created immediately.

I tested it with a 5TB volume.

NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda      8:0    0    5T  0 disk 
├─sda1   8:1    0  200M  0 part /boot/efi
├─sda2   8:2    0    1G  0 part /boot
├─sda3   8:3    0  3.9G  0 part [SWAP]
├─sda4   8:4    0   50G  0 part /
└─sda5   8:5    0    5T  0 part /home

Ver 0.11.6, it takes less than 0.1 seconds to create a 500GB COW file, for example.
Ver0.11.6.txt

10/09/2024 06:35:33.630 ** datto file does not exist.
10/09/2024 06:35:34.098 ** dbdctl setup-snapshot /dev/sda5 /home/.datto0 0
10/09/2024 06:35:34.105 507G .datto0 　------------->10% COW file is created immediately
10/09/2024 06:35:34.117 ** Created snapshot /dev/sda5.
10/09/2024 06:35:34.121 ** sleep 5.
10/09/2024 06:35:39.129 ** dbdctl destroy 0.
10/09/2024 06:35:39.154 ** destroy fin.

Ver 0.11.7 and later, it takes 30 minutes to create a 500GB COW file.
Ver0.11.8.txt

10/09/2024 05:44:24.022 ** datto file does not exist.
10/09/2024 05:44:24.902 ** dbdctl setup-snapshot /dev/sda5 /home/.datto0 0
10/09/2024 05:44:25.034 ** datto file does not exist.
10/09/2024 05:44:26.040 342M .datto0
10/09/2024 05:44:27.051 921M .datto0
            :
            :
10/09/2024 06:15:08.194 506G .datto0
10/09/2024 06:15:10.888 507G .datto0 　------------->It takes 30 minutes to create 10% of the COW file
10/09/2024 06:15:13.268 ** Created snapshot /dev/sda5.
10/09/2024 06:15:13.926 ** sleep 5.
10/09/2024 06:15:18.931 ** dbdctl destroy 0.
10/09/2024 06:15:20.648 ** destroy fin.

[iamandrii]

Hi @yito24. Thanks for raising this again. I think I know the reason (starting from 0.11.7 we are using our fallocate instead of kernel one). We will proceed with this bug in one of our next releases. Have a nice day =)

[yito24]

Hi @iamandrii,

If this issue is fixed, the quality will be better.
I look forward to it being fixed.

[yito24]

Hi @iamandrii,

I was download "dattobd-real-fallocate-fix" and tested it.
I have confirmed that the COW is created in a similar amount of time as in version 0.11.6.