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Date:      Sun, 11 Dec 2022 09:52:48 +0300
From:      Artem Kuchin <artemkuchin76@gmail.com>
To:        freebsd-fs@FreeBSD.org
Subject:   Everchanging bytes at the end of mirror disks
Message-ID:  <85c5a64c-915e-d790-e617-c94f3fb7cd9a@gmail.com>

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Hello!

I am writing a small utility for myseld and part of it is comparing 
gmirror disks. After running some tests i realized that some bytes at 
the very end of disks are constantly changing.

For example to runs from the compator:

Difference between /dev/ada0p3 [26,0x1A]  and /dev/ada1p3 [95,0x5F] at 
position 495812794495
Difference between /dev/ada0p3 [217,0xD9]  and /dev/ada1p3 [107,0x6B] at 
position 495812794496
Difference between /dev/ada0p3 [151,0x97]  and /dev/ada1p3 [0,0x0] at 
position 495812794497
Difference between /dev/ada0p3 [180,0xB4]  and /dev/ada1p3 [64,0x40] at 
position 495812794498
Difference between /dev/ada0p3 [96,0x60]  and /dev/ada1p3 [126,0x7E] at 
position 495812794499
Difference between /dev/ada0p3 [89,0x59]  and /dev/ada1p3 [239,0xEF] at 
position 495812794500
Difference between /dev/ada0p3 [234,0xEA]  and /dev/ada1p3 [93,0x5D] at 
position 495812794501
Difference between /dev/ada0p3 [166,0xA6]  and /dev/ada1p3 [76,0x4C] at 
position 495812794502

Difference between /dev/ada0p3 [185,0xB9]  and /dev/ada1p3 [233,0xE9] at 
position 495812794495
Difference between /dev/ada0p3 [145,0x91]  and /dev/ada1p3 [2,0x2] at 
position 495812794496
Difference between /dev/ada0p3 [105,0x69]  and /dev/ada1p3 [3,0x3] at 
position 495812794497
Difference between /dev/ada0p3 [55,0x37]  and /dev/ada1p3 [166,0xA6] at 
position 495812794498
Difference between /dev/ada0p3 [23,0x17]  and /dev/ada1p3 [99,0x63] at 
position 495812794499
Difference between /dev/ada0p3 [35,0x23]  and /dev/ada1p3 [144,0x90] at 
position 495812794500
Difference between /dev/ada0p3 [118,0x76]  and /dev/ada1p3 [212,0xD4] at 
position 495812794501
Difference between /dev/ada0p3 [30,0x1E]  and /dev/ada1p3 [149,0x95] at 
position 495812794502

End of disk is 495812794880, so, the last changing bytes if 378 bytes 
before the end.

I am most likely missing some information about FS/mirror organization. 
Below is the filesystem configuration.

However, if there a part which changes on both disks independently how 
do i get its offset and size,

so i will not compare it.  Thank you.


# gmirror status
        Name    Status  Components
mirror/swap  COMPLETE  ada0p2 (ACTIVE)
                        ada1p2 (ACTIVE)
mirror/root  COMPLETE  ada0p3 (ACTIVE)
                        ada1p3 (ACTIVE)

# gpart show
=>       34  976773101  ada0  GPT  (466G)
          34        128     1  freebsd-boot  (64K)
         162    8388608     2  freebsd-swap  (4.0G)
     8388770  968384365     3  freebsd-ufs  (462G)

=>       34  976773101  ada1  GPT  (466G)
          34        128     1  freebsd-boot  (64K)
         162    8388608     2  freebsd-swap  (4.0G)
     8388770  968384365     3  freebsd-ufs  (462G)







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