On 07/02/2010 20:10, Christo Kutrovsky wrote:
Eric,
I am confused. What's difference between:
- turning off slogs (via logbias)
slog = Separate Log Device
slog != ZIL
There is zero or one slog (maybe mirrored) per pool.
Every dataset has its own ZIL.
The logbias property provides a hint to wither or not
the intent is throughput or latency. For latency if
there is a slog present then the ZIL is likely to get
most of its blocks allocated from the slog.
The reason this property exists is to allow for a mix
of different datasets per pool where some of them latency
matters more and others throughput matters more.
- turning off ZIL (via kernel tunable)
That disables the ZIL for *all* datasets on *all* pools
on the system. Doing this means that for NFS client or
other applications (maybe local) that rely on the POSIX
synchronus requirements of fsync they may see data loss
on a crash. Note that the ZFS pool is still consistent on
disk but the application that is flushing writes synchonusly
may have missing data on recovery from the crash.
NEVER turn off the ZIL other than for testing on dummy data
wither or not the ZIL is your bottle neck. NEVER turn off
the ZIL on live data pools.
Isn't that similar, just one is more granular?
Totally different.
--
Darren J Moffat
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