l2arc slows write performance (but is still well under the SSDs write bw)

[cwedgwood]

v3.4.10

l2arc 120GB Intel SSD on an uncontended SATA device

raidz of 5x2TB of an lsi sas controller (so different pci-e devices)

• zil (off sata, no activitity)

When the cache device is attached write performance is badly impacted, though it's not clear why.

It's not a bw limitation of the SSD, whilst though could in some cases affect things, in this case the performance went from 21MB/s to 43MB/s ...

(other people have reported similar things when i google., though i couldn't find any way to bypass the l2arc for large writes if indeed suc ha think makes sense)

[cwedgwood]

There are so many unknowns here. Closing for now until those are better isolated and quantified.

[behlendorf]

For the sake of documentation let me add that the L2ARC is never written to in the write path. It is asynchronously populated by the l2arc_feed_thread, so I wouldn't expect adding this device to have an impact. However, the issue might be caused by other contention, for example in the ARC. This is something we could consider looking in to one we get to the real performance work.