gmirror: DIRTY flag?

I have some questions about gmirror "list" report:

1) What does the "DIRTY" flag mean? Anything bad?
2) When (and how) does the SyncID field change?


t1# gmirror list
Geom name: netmirror
Components: 2
Balance: round-robin
Slice: 4096
Flags: NONE
SyncID: 1
ID: 3859137981
Providers:
1. Name: mirror/netmirror
    Mediasize: 268434944 (256M)
    Sectorsize: 512
    Mode: r1w1e0
Consumers:
1. Name: ad2s1e
    Mediasize: 268435456 (256M)
    Sectorsize: 512
    Mode: r1w1e1
    State: ACTIVE
    Priority: 0
    Flags: DIRTY
    SyncID: 1
    ID: 1417083766
2. Name: ggate0
    Mediasize: 268435456 (256M)
    Sectorsize: 512
    Mode: r1w1e1
    State: ACTIVE
    Priority: 1
    Flags: DIRTY
    SyncID: 1
    ID: 899580387

Geom name: netmirror.sync

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