On Jan 4, 2010, at 7:08 PM, Brad wrote:
Hi Adam,
From your the picture, it looks like the data is distributed evenly
(with the exception of parity) across each spindle then wrapping
around again (final 4K) - is this one single write operation or two?
| P | D00 | D01 | D02 | D03 | D04 | D05 | D06 | D07 | <---------
one write op??
| P | D08 | D09 | D10 | D11 | D12 | D13 | D14 | D15 | <---------
one write op??
One physical write op per vdev because the columns will likely be
coalesced at the vdev. Obviously, one physical write cannot span
multiple vdevs.
For a stripe configuration, is this would it would like look for 8K?
| D00 D01 D02 D03 D04 D05 D06 D07 D08 |
| D09 D10 D11 D12 D13 D14 D15 D16 D17 |
No. It is very likely the entire write will be to one vdev. Again,
this is
dynamic striping, not RAID-0. RAID-0 is defined by SNIA as "A disk array
data mapping technique in which fixed-length sequences of virtual disk
data addresses are mapped to sequences of member disk addresses
in a regular rotating pattern." In ZFS, there is no "fixed-length
sequence."
The next column is chosen approximately every MB or so. You get the
benefit of sequential access to the media, with the stochastic spreading
across vdevs as well.
When you have multiple top-level vdevs, such as multiple mirrors or
multiple raidz sets, then you get the ~ 1MB spread across the top level
and the normal allocations in the sets. In other words, any given
record
should be in one set. Again, this limits hyperspreading and allows you
to scale to very large numbers of disks. It seems to work reasonably
well in practice. I attempted to describe this in pictures for my ZFS
tutorials. You can be the judge, and suggestions are always welcome.
See slide 27 at
http://www.slideshare.net/relling/zfs-tutorial-usenix-lisa09-conference
[for the alias, I've only today succeeded in uploading the slides to
slideshare... been trying off and on for more than a month :-(]
-- richard
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