[9fans] Re: File Server extention for mirrored devices

[9fans] Re: File Server extention for mirrored devices

[G.David]

>I would like to propose the following extension to fsconfig(8):
>
>{ device... }
>  A pseudo-device formed by mirroring corresponding blocks of the
>  devices in the list.  The size of the result is the number of
>  blocks on the smallest device.

This has been done.  Because of the available functionality, the
mirror is N-way.  The performance is the same as a single device
by itself (no round-robin on the reads nor sync-writes necessary
for a volume version number.)  If you need performance, use striping.
If you want both performance and safety, mirror a stripe.

Write errors take a member off line.  Read errors cause the read
from the good member to re-write the bad (for bit rot and block
remap.)  Version numbers are used to determine if partners of a
mirror are in sync at boot time.  If not, the out of sync members
are recovered from the good ones while the system is running.
If the system crashed, just reboot and run check like normal.  The
mirror system will do the right thing.

If a drive is replaced (after power-off) and the system is restarted,
the mirror system detects the new device, and after making sure it
is at least as large as the mirror, brings it into service and starts
recovery.

The device also handles the writing of the config block at any time.
This is good place to save the config block even if you don't use
mirroring for anything else.

This functioality is available in Plan9, not Brazil.  :->

David Butler
gdb@dbSystems.com