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On Mon, 11 Jan 2010 19:45, fjwcash@ wrote:
On Mon, Jan 11, 2010 at 4:24 PM, Dan Naumov <dan.nau...@gmail.com> wrote:
On Tue, Jan 12, 2010 at 1:29 AM, K. Macy <km...@freebsd.org> wrote:
If performance is an issue, you may want to consider carving off a
partition
on that SSD, geli-fying it, and using it as a ZIL device. You'll
probably
see a marked performance improvement with such a setup.
That is true, but using a single device for a dedicated ZIL is a huge
no-no, considering it's an intent log, it's used to reconstruct the
pool in case of a power failure for example, should such an event
occur at the same time as a ZIL provider dies, you lose the entire
pool because there is no way to recover it, so if ZIL gets put
"elsewhere", that elsewhere really should be a mirror and sadly I
don't see myself affording to use 2 SSDs for my setup :)
This is false. The ZIL is used for journalling synchronous writes. If
your ZIL is lost you will lose the data that was written to the ZIL,
but not yet written to the file system proper. Barring disk
corruption, the file system is always consistent.
-Kip
Ok, lets assume we have a dedicated ZIL on a single non-redundant
disk. This disk dies. How do you remove the dedicated ZIL from the
pool or replace it with a new one? Solaris ZFS documentation indicates
that this is possible for dedicated L2ARC - you can remove a dedicated
l2arc from a pool at any time you wish and should some IO fail on the
l2arc, the system will gracefully continue to run, reverting said IO
to be processed by the actual default built-in ZIL on the disks of the
pool. However the capability to remove dedicated ZIL or gracefully
handle the death of a non-redundant dedicated ZIL vdev does not
currently exist in Solaris/OpenSolaris at all.
That has been implemented in OpenSolaris, do a search for "slog removal".
It's in a much newer zpool version than 13, though.
What I have seen more often by users is taking the usage of slog/ZIL
wrong. For instance dedicating a whole SSD or another HDD as the slog.
Your slog/ZIL only has to be big enough to handle 10 seconds of synchronous
writes before it flushes. A recommended ZIL from Sun Micro is 128MB but
you may not even see that fully used for general purpose cases.
I had is to dedicate a partition on the same disk that the pool is on and
adding another ZIL vdev from another disk in the system. Results of this
imply that if the off-disk ZIL dies for some stupid reason it falls back
to the one that rests on the same disk as the pool and allows to replace
the off-disk ZIL with something else.
PS: Save your disk space and use 256MB thumb drives. you can easily get 16
of those at your local Walmart and have a priceless light show for a
romantic dinner with the wife.
:)
- --
Mon Jan 11 22:31:17 2010
jhell
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