Bob Friesenhahn wrote:
My own reliability concerns regarding a "SAN" are due to the big-LUN
that SAN hardware usually emulates and not due to communications in the
"SAN". A big-LUN is comprised of multiple disk drives. If the SAN
storage array has an error, then it is possible that the data on one of
these disk drives will be incorrect, and it will be hidden somewhere in
that big LUN. The data could be old data rather than just being
"corrupted". Without redundancy ZFS will detect this corruption but
will be unable to repair it. The difference from UFS is that UFS might
not even notice the corruption, or fsck will just paper it over. UFS
filesystems are usually much smaller than ZFS pools.
There are performance concerns when using a big-LUN because ZFS won't be
able to intelligently schedule I/O for multiple drives, so performance
is reduced.
Also, ZFS does things like putting the ZIL data (when not on a dedicated
device) at the outer edge of disks, that being faster. When you have a
LUN which doesn't map on to standard performance profile of a disk, this
optimsation is lost.
I give talks on ZFS to Enterprise customers, and this area is something
I cover. Where possible, give ZFS visibility of redundancy, and as many
LUNs as you can. However, we have to recognise that this isn't always
possible. In many enterprises, storage is managed by separate teams from
servers (this is a legal requirement in some industry sectors in some
countries, typically finance), often with very little cooperation
between teams, indeed even rivalry. If we said ZFS _had_ to handle lots
of LUNs and the data redundancy, it would never get through many data
centre doors, so we do have to work in this environment.
Even where customers can't make use of some of the features such as self
healing data corruptions, I/O scheduling, etc, because of their company
storage infrastructure limitations, there's still a ton of other
goodness in there too with ease of creating filesystems, snapshots, etc.
and we will at least let them know when their multi-million dollar
storage system silently drops a bit, which they tend to far more often
than most customers realise.
--
Andrew
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