dmsetup：持久元數據儲存在哪裡？

dmsetup快照文件說：

<persistent?> is P (Persistent) or N (Not persistent - will not survive
after reboot).  O (Overflow) can be added as a persistent store option
to allow userspace to advertise its support for seeing "Overflow" in the
snapshot status.  So supported store types are "P", "PO" and "N".

The difference between persistent and transient is with transient
snapshots less metadata must be saved on disk - they can be kept in
memory by the kernel.

這些持久數據儲存在哪裡？

dmsetup持久快照設備與瞬態快照設備的第一個塊中的數據存在差異：

鑑於這些設備：

$ losetup
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                         DIO
/dev/loop1         0      0         0  0 /home/var/ravi/tmp/issue/snap-dev   0
/dev/loop0         0      0         0  0 /home/var/ravi/tmp/issue/base-dev   0

和一個最初歸零的快照設備支持文件：

$ od -xc snap-dev
0000000    0000    0000    0000    0000    0000    0000    0000    0000
        \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
3751613000

以下是使用非持久N標誌時發生​​的情況：

$ sudo dmsetup -v create snapdev --table '0 8 snapshot /dev/loop0 /dev/loop1 N 1'
Name:              snapdev
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      254, 5
Number of targets: 1

$ od -xc snap-dev
0000000    0000    0000    0000    0000    0000    0000    0000    0000
        \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
3751613000

請注意，備份文件未更改 - 仍然是\0字節。

現在，再次嘗試使用P標誌以保持持久性：

$ sudo dmsetup remove snapdev
$ sudo dmsetup -v create snapdev --table '0 8 snapshot /dev/loop0 /dev/loop1 P 1'
Name:              snapdev
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      254, 5
Number of targets: 1

$ od -xc snap-dev
0000000    6e53    7041    0001    0000    0001    0000    0001    0000
         S   n   A   p 001  \0  \0  \0 001  \0  \0  \0 001  \0  \0  \0
0000020    0000    0000    0000    0000    0000    0000    0000    0000
        \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
3751613000

在這種情況下，設備的第一個字節是SnAp\001.

---

我的猜測是持久數據儲存在快照設備本身的第一個或多個塊中。

引用自：https://unix.stackexchange.com/questions/396966