Adam Leventhal wrote:
1) How quickly and by what trigger is data flushed from the SLOG to
primary storage? (I setup a test once just using normal disks and I
didn't see any data migration occurring.)
Data isn't really flushed from the slog to primary storage -- indeed, the
slog is never read from during normal operation. Instead data is streamed
to primary storage asynchronously. This should happen within 5-10 seconds
at regular intervals as transaction groups are closed.
If data is never read from a slog device, does that mean that inserted
data is immediately available in ARC, and cannot be evacuated from ARC
until offloaded from the slog device to primary storage? Or does it
mean that reads cannot occur until the transaction group is closed and
it's been pushed to primary?
2) How can I monitor SLOG fullness in order to determine whether
additional capacity is required?
Rather than monitoring fullness, I'd suggest you look at the datarate to the
slog device.
I've tried measuring this data rate, and it was not very accurate, using
zpool iostat. I was expecting to be able to use a kstat target to get
data, such as "kstat zfs:arcstats", but was unable to find such a thing.
Adam
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