On 12/20/2010 11:56 AM, Mark Sandrock wrote:
Erik,
just a hypothetical what-if ...
In the case of resilvering on a mirrored disk, why not take a snapshot, and then
resilver by doing a pure block copy from the snapshot? It would be sequential,
so long as the original data was unmodified; and random access in dealing with
the modified blocks only, right.
After the original snapshot had been replicated, a second pass would be done,
in order to update the clone to 100% live data.
Not knowing enough about the inner workings of ZFS snapshots, I don't know why
this would not be doable. (I'm biased towards mirrors for busy filesystems.)
I'm supposing that a block-level snapshot is not doable -- or is it?
Mark
Snapshots on ZFS are true snapshots - they take a picture of the current
state of the system. They DON'T copy any data around when created. So, a
ZFS snapshot would be just as fragmented as the ZFS filesystem was at
the time.
The problem is this:
Let's say I write block A, B, C, and D on a clean zpool (what kind, it
doesn't matter). I now delete block C. Later on, I write block E.
There is a probability (increasing dramatically as times goes on), that
the on-disk layout will now look like:
A, B, E, D
rather than
A, B, [space], D, E
So, in the first case, I can do a sequential read to get A & B, but then
must do a seek to get D, and a seek to get E.
The "fragmentation" problem is mainly due to file deletion, NOT to file
re-writing. (though, in ZFS, being a C-O-W filesystem, re-writing
generally looks like a delete-then-write process, rather than a modify
process).
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
Timezone: US/Pacific (GMT-0800)
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