LICON, RAY (ATTPB) wrote:
Thanks for the reply.
In many situations, the hardware design isn't up to me and budgets tend
to dictate everything these days. True, nobody wants to swap, but the
question is "if" you had to -- what design serves you best. Independent
swap slices or putting it all under control of zfs.
It depends why you need to swap, i.e. why are you using more memory than
you have, and is your working set size bigger than memory (thrashing),
or is swapping likely to be just a once-off event or infrequently repeated?
You probably need to forget most of what you learned about swapping 25
years ago, when systems routinely swapped, and technology was very
different. Disks have got faster over that period, probably of the order
100 times faster. However, CPUs have got 100,000 times faster, so in
reality a disk looks to be 1000 times slower from the CPU's standpoint
than it did 25 years ago. This means that CPU cycles lost due to
swapping will appear to have a proportionally much more dire effect on
performance than they did many years back.
There are lots more options available today than there were when systems
routinely swapped. A couple of examples that spring to mind...
ZFS has been explicitly designed to swap it's own cache data, only we
don't call it swapping - we call it an L2ARC or ReadZilla. So if you
have a system where the application is going to struggle with main
memory, you might configure ZFS to significantly reduce it's memory
buffer (ARC), and instead give it an L2ARC on a fast solid state disk.
This might result in less performance degradation in some systems where
memory is short, depending heavily on the behaviour of the application.
If you do have to go with brute force old style swapping, then you might
want to invest in solid state disk swap devices, which will go some way
towards reducing the factor of 1000 I mentioned above. (Take note of
aligning swap to the 4k flash i/o boundaries.)
Probably lots of other possibilities too, given more than a couple of
minutes thought.
--
Andrew
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
