On May 20, 2010, at 7:17 PM, Ragnar Sundblad <ra...@csc.kth.se> wrote:
On 21 maj 2010, at 00.53, Ross Walker wrote:
On May 20, 2010, at 6:25 PM, Travis Tabbal <tra...@tabbal.net> wrote:
use a slog at all if it's not durable? You should
disable the ZIL
instead.
This is basically where I was going. There only seems to be one
SSD that is considered "working", the Zeus IOPS. Even if I had the
money, I can't buy it. As my application is a home server, not a
datacenter, things like NFS breaking if I don't reboot the clients
is a non-issue. As long as the on-disk data is consistent so I
don't have to worry about the entire pool going belly-up, I'm
happy enough. I might lose 30 seconds of data, worst case, as a
result of running without ZIL. Considering that I can't buy a
proper ZIL at a cost I can afford, and an improper ZIL is not
worth much, I don't see a reason to bother with ZIL at all. I'll
just get a cheap large SSD for L2ARC, disable ZIL, and call it a
day.
For my use, I'd want a device in the $200 range to even consider
an slog device. As nothing even remotely close to that price range
exists that will work properly at all, let alone with decent
performance, I see no point in ZIL for my application. The
performance hit is just too severe to continue using it without an
slog, and there's no slog device I can afford that works properly,
even if I ignore performance.
Just buy a caching RAID controller and run it in JBOD mode and have
the ZIL integrated with the pool.
A 512MB-1024MB card with battery backup should do the trick. It
might not have the capacity of an SSD, but in my experience it
works well in the 1TB data moderately loaded range.
Have more data/activity then try more cards and more pools,
otherwise pony up the $$$$ for a capacitor backed SSD.
It - again - depends on what problem you are trying to solve.
If the RAID controller goes bad on you so that you loose the
data in the write cache, your file system could be in pretty bad
shape. Most RAID controllers can't be mirrored. That would hardly
make a good replacement for a mirrored ZIL.
As far as I know, there is no single silver bullet to this issue.
That is true, and there at finite budgets as well and as all things in
life one must make a trade-off somewhere.
If you have 2 mirrored SSDs that don't support cache flush and your
power goes out your file system will be in the same bad shape.
Difference is in the first place you paid a lot less to have your data
hosed.
-Ross
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