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doc: explain zfs snapshots + architecture
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Thomas Schoebel-Theuer
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59
docu/images/raid-lvm-architecture.fig
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59
docu/images/raid-lvm-architecture.fig
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#FIG 3.2 Produced by xfig version 3.2.5c
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Landscape
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Center
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Metric
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A4
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100.00
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1200 2
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6 270 2610 630 2880
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 450 2700 180 90 450 2700 630 2610
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 450 2790 180 90 450 2790 630 2700
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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270 2700 270 2790
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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630 2700 630 2790
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-6
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6 1170 2610 1530 2880
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 1350 2700 180 90 1350 2700 1530 2610
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 1350 2790 180 90 1350 2790 1530 2700
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1170 2700 1170 2790
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1530 2700 1530 2790
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-6
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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720 2250 540 2610
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1080 2250 1260 2610
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2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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450 1980 1350 1980 1350 2250 450 2250 450 1980
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2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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450 270 1350 270 1350 720 450 720 450 270
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2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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450 1620 1350 1620 1350 1890 450 1890 450 1620
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2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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450 1260 1350 1260 1350 1530 450 1530 450 1260
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2 2 1 1 0 -1 50 -1 -1 4.000 0 0 -1 0 0 5
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450 810 1350 810 1350 1080 450 1080 450 810
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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900 720 900 810
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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900 1080 900 1260
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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900 1530 900 1620
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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900 1890 900 1980
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4 1 0 50 -1 2 15 0.0000 4 45 180 900 2790 ...\001
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4 1 0 50 -1 -1 10 0.0000 4 135 780 900 3060 48 spindles\001
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4 1 0 50 -1 -1 10 0.0000 4 105 195 900 450 zfs\001
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4 1 0 50 -1 -1 10 0.0000 4 135 660 900 630 snapshots\001
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4 1 0 50 -1 2 15 0.0000 4 45 180 900 2430 ...\001
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4 1 0 50 -1 -1 10 0.0000 4 105 405 900 2160 RAID\001
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4 1 0 50 -1 -1 10 0.0000 4 120 660 900 1800 pvs + vgs\001
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4 1 0 50 -1 -1 10 0.0000 4 135 735 900 990 replication\001
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4 1 0 50 -1 -1 10 0.0000 4 105 195 900 1440 lvs\001
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4 1 0 50 -1 -1 10 0.0000 4 105 270 270 1440 10x\001
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4 1 0 50 -1 -1 10 0.0000 4 105 270 270 990 10x\001
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4 1 0 50 -1 -1 10 0.0000 4 105 270 270 540 10x\001
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47
docu/images/zpool-architecture.fig
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docu/images/zpool-architecture.fig
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#FIG 3.2 Produced by xfig version 3.2.5c
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Landscape
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Center
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Metric
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100.00
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Single
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-2
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1200 2
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6 270 2610 630 2880
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 450 2700 180 90 450 2700 630 2610
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 450 2790 180 90 450 2790 630 2700
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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270 2700 270 2790
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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630 2700 630 2790
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-6
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6 1170 2610 1530 2880
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 1350 2700 180 90 1350 2700 1530 2610
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1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 1350 2790 180 90 1350 2790 1530 2700
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1170 2700 1170 2790
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1530 2700 1530 2790
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-6
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6 90 540 450 720
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4 1 0 50 -1 -1 10 0.0000 4 105 270 270 720 10x\001
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-6
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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720 2250 540 2610
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2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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1080 2250 1260 2610
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2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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450 270 1350 270 1350 2250 450 2250 450 270
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2 1 1 1 0 7 50 -1 -1 4.000 0 0 -1 0 0 2
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450 1260 1350 1260
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4 1 0 50 -1 2 15 0.0000 4 45 180 900 2790 ...\001
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4 1 0 50 -1 -1 10 0.0000 4 135 780 900 3060 48 spindles\001
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4 1 0 50 -1 -1 10 0.0000 4 135 390 900 1620 zpool\001
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4 1 0 50 -1 -1 10 0.0000 4 135 885 900 1980 functionality\001
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4 1 0 50 -1 -1 10 0.0000 4 120 660 900 1800 pvs + vgs\001
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4 1 0 50 -1 -1 10 0.0000 4 105 195 900 450 zfs\001
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4 1 0 50 -1 -1 10 0.0000 4 135 660 900 630 snapshots\001
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4 1 0 50 -1 -1 10 0.0000 4 105 495 900 810 +RAID\001
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4 1 0 50 -1 2 15 0.0000 4 45 180 900 2430 ...\001
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4 1 0 50 -1 20 8 0.0000 4 135 810 900 1350 interface\001
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4 1 0 50 -1 20 8 0.0000 4 135 720 900 1260 internal\001
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@ -2525,6 +2525,189 @@ The last item means that ZFS by itself does not protect against amok-running
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enterprise-critical applications.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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Notice that zfs snapshots can be combined with DRBD or MARS, because zfs
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snapshots are residing at
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\emph on
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filesystem
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\emph default
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layer, while DRBD / MARS replicas are located at
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\emph on
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block
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\emph default
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layer.
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Just create your zpools at the
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\emph on
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top
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\emph default
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of DRBD or MARS virtual devices, and import / export them
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\emph on
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individually
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\emph default
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upon handover / failover of each LV.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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There is a
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\series bold
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\emph on
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fundamental
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\series default
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\emph default
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difference between zpools and classical RAID / LVM stacked architectures.
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Some zfs advocates are propagating zpools as a replacement for both RAID
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and LVM.
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However, there is a
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\series bold
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massive difference
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\series default
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in architecture, as explained in the following example (10 logical resources
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over 48 physical spindles), achieving practically the
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\series bold
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\emph on
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same
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\series default
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zfs snapshot functionality
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\emph default
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from a user's perspective, but in a different way:
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\end_layout
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\begin_layout Standard
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\noindent
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\align center
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\begin_inset Graphics
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filename images/raid-lvm-architecture.fig
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height 6cm
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\end_inset
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\begin_inset Graphics
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filename images/zpool-architecture.fig
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height 6cm
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\end_inset
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\end_layout
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\begin_layout Standard
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\noindent
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When RAID functionality is executed by zfs, it will be located at the
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\emph on
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top
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\emph default
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of the hierarchy.
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On one hand, this easily allows for different RAID levels for each of the
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10 different logical resources.
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On the other hand, this
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\emph on
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exposes
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\emph default
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the
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\series bold
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physical spindle configuration
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\series default
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to the topmost filesystem layer (48 spindles in this example).
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There is no easy way for replication of these
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\emph on
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physical properties
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\emph default
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in a larger / heterogenous distributed system, e.g.
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when some hardware components are replaced over a longer period of time
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(hardware lifecycle, or LV Football as explained in chapter
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "chap:LV-Football"
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\end_inset
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).
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Essentially, only replication of
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\emph on
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logical
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\emph default
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structures like snapshots remains as the only reasonable option, with its
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drawbacks as explained above.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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There is another argument: zfs tries to
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\emph on
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hide
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\emph default
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its internal structures and interfaces from the sysadmins, forming a more
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or less
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\series bold
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monolithic
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Some sysadmins acting as zfs advocates are reclaiming this as an advantage,
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because they need to understand only a single tool for managing
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\begin_inset Quotes eld
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\end_inset
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everything
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\begin_inset Quotes erd
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\end_inset
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.
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However, this is a short-sighted argument when it comes to
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\emph on
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true
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\emph default
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flexibility as offered by a component-based system, where multiple types
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of hardware / software RAID, multiple types of LVM functionality, and much
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more can be almost orthogonally combined in a very flexible way.
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\end_layout
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\end_inset
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architecture
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\series default
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as seen from outside.
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This violates the classical
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\emph on
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layering rules
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\emph default
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from Dijkstra.
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In contrast, classical LVM-based configurations are
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\series bold
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component oriented
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\series default
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, according to the
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\series bold
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Unix philosophy
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\series default
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.
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\end_layout
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\begin_layout Section
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Local vs Centralized Storage
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\begin_inset CommandInset label
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