Andrew wrote:
Jeff Bonwick wrote:
For a synchronous write to a pool with mirrored disks, does the write
unblock after just one of the disks' write caches is flushed, or
only after all of the disks' caches are flushed?
The latter. We don't consider a write to be committed until the data is
on stable storage at full replication.
[snip]
That makes sense, but there's a point at which ZFS must abandon this
strategy; otherwise, the malfunction of one disk in a 3-way mirror could
halt the entire system, when what's probably desired is for the system to
keep running in degraded mode with only 2 remaining functional disks in
the mirror.
But then of course there would be the problem of divergent disks in a
mirror; suppose there's a system with one pool on a pair of mirrored
disks, and system root is on that pool. The disks are external, with
interface cables running across the room. The system is running fine
until my dog trips over the cable for disk #2. Down goes disk #2, and the
system continues running fine, with a degraded pool, and during operation
continues modifying various files. Later, the dog chews through the cable
for disk #1. Down goes the system. I don't have a spare cable, so I just
plug in disk #2, and restart the system. The system continues running
fine, with a degraded pool, and during operation continues modifying
various files. I go to the store to buy a new cable for disk #1, and when
I come back, I trip over the cable for disk #2. Down goes the system. I
plug #2 back in, replace the cable for #1, and restart the system. At
this point, the system comes up with its root on a pool with divergent
mirrors, and... ?
Wow, you must be the unluckiest person ever! And such a strong dog.....
So when the system comes back up, your uberblock and your ditto blocks
will be examined, and those which have incorrect checksums will be
detected and fixed.
James C. McPherson
--
Solaris Datapath Engineering
Storage Division
Sun Microsystems
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