Paul B. Henson wrote:
So I was looking into the boot flash feature of the newer x4540, and
evidently it is simply a CompactFlash slot, with all of the disadvantages
and limitations of that type of media. The sun deployment guide recommends
minimizing writes to a CF boot device, in particular by NFS mounting /var
from a different server, disabling swap or swapping to a different device,
and doing all logging over the network.
argv. So we've had the discussion many times over the past 4 years about
why these "recommendations" are largely bogus. Alas, once published,
they seem to live forever.
The presumption is that you are using UFS for the CF, not ZFS.
UFS is not COW, so there is a potential endurance problem for
blocks which are known to be rewritten many times. ZFS will not
have this problem, so if you use ZFS root, you are better served by
ignoring the previous "advice."
For additional background, if you worry about UFS and endurance,
then you want to avoid all writes, because metadata is at fixed
locations, and you could potentially hit endurance problems at those
locations. Some people think that /var collects a lot of writes, and it
might if you happen to be running a high-volume e-mail server using
sendmail. Since almost nobody does that in today's internet, the risk
is quite small.
The second thought was that you will be swapping often and therefore
you want to avoid the endurance problem which affects swap (where
the swap device is raw, not a file system). In practice, if you have a
lot of swap activity, then your performance will stink and you will
be more likely to actually buy some RAM to solve the problem. Also,
most modern machines are overconfigured for RAM, so the actual
swap device usage for modern machines is typically low. I had some
data which validated this assumption, about 4 years ago. It is easy to
monitor swap usage, so see for yourself if your workload does a lot
of writes to the swap device. For OpenSolaris (enterprise support
contracts now available!) which uses ZFS for swap, don't worry, be
happy.
In short, if you use ZFS for root, ignore the warnings.
Not exactly a configuration I would
prefer. My sales SE said most people weren't utilizing the CF boot feature.
The concept is nice, but an implementation with SSD quality flash rather
than basic CF (also, preferably redundant devices) would have been better.
It depends on the market. In telco, many people use CF for boot
because they are much more reliable under much more diverse
environmental conditions than magnetic media.
If I had an x4540 (which I don't, unfortunately, we picked up a half dozen
x4500's just before they were end of sale'd), what I think would be
interesting to do would be install two of the 32GB SSD disks in the boot
slots, use a 1-5GB sliced mirror as a slog, and the remaining 27-31GB as a
sliced mirrored root pool.
5 GBytes seems pretty large for a slog, but yes, I think this is a good
idea.
From what I understand you don't need very much
space for an effective slog, and SSD's don't have the write failure
limitations of CF.
CFs designed for the professional photography market have better
specifications than CFs designed for the consumer market.
Also, the recommendation for giving ZFS entire discs
rather than slices evidently isn't applicable to SSD's as they don't have a
write cache. It seems this approach would give you a blazing fast slog, as
well as a redundant boot mirror without having to waste an additional two
SATA slots.
This is not an accurate statement. Enterprise-class SSDs (eg. STEC Zeus)
have DRAM write buffers. The Flash Mini-DIMMs Sun uses also have
DRAM write buffers. These offer very low write latency for slogs.
-- richard
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