Bob Friesenhahn wrote:
On Tue, 15 Jun 2010, Arne Jansen wrote:
In case of a power failure I will likely lose about as many writes as
I do with SSDs, a few milliseconds.
I agree with your concerns, but the data loss may span as much as 30
seconds rather than just a few milliseconds.
Wait, I'm talking about using SSD for ZIL vs. using a dedicated hard drive
for ZIL which is configured to ignore cache flushes. Do you say I can lose
30 seconds also if I use a badly behaving SSD?
Using an SSD as the ZIL allows zfs to turn a synchronous write into a
normal batched async write which is scheduled for the next TXG. Zfs
intentionally postpones writes.
Without the SSD, zfs needs to write to an intent log in the main pool
(consuming precious IOPS) or write directly to the main pool (consuming
precious response latency). Battery-backed RAM in the adaptor card or
storage array can do almost as well as the SSD as long as the amount of
data does not overrun the limited write cache.
Bob
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