02.07.2011 16:07

Snapshot-Struktur

dmsetup table | grep 'var'
vg1-var-real: 0 15622144 linear 253:0 27328896
vg1-var.snap: 0 15622144 snapshot 253:6 253:7 P 8
vg1-var.snap-cow: 0 3121152 linear 253:0 109330816
vg1-var: 0 15622144 snapshot-origin 253:6

Wie wirken diese vier Devices nun zusammen? Ich übertrage die Ausführungen von Richard WM Jones in seinem Blog-Beitrag "How LVM does snapshots".

       vg1-var               vg1-var.snap
          |                     |   |
          |     +---------------+   |
          v     v                   v
       vg1-var-real          vg1-var.snap-cow

Ich zitiere aus dem o.g. Artikel:

F13x64snap-cow at the bottom right is the actual storage used for the snapshot exception list. It is just a plain linear mapping of some blocks from the underlying block device. F13x64snap is the virtual device. When read, the read consults the exception list in the snapshot cow, and if not there, consults the real device. Writes to F13x64snap go to the exception list. Finally, writes to the virtual origin device F13x64 go to the snapshot cow (or snapshot cows plural). There is no explicit connection here — in fact it goes via a hash table stored in kernel memory."

Ersetzen Sie beim Lesen einfach F13x64 durch vg1-var.


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